If there was ever a museum that doesn’t need to hold loan exhibitions, it’s the Frick Collection. Routinely described as "the most perfect" small museum in America (though personally I’d place the Clark Art Institute in Williamstown right alongside it), the Frick has graciously hosted numerous small shows that might otherwise have bypassed Manhattan. With rare exceptions, like the retrospective of the drawings of Jean-Baptiste Greuze in 2002, most of these special exhibitions have not lingered long in the memory. But the Frick’s current show of portraits of the 15th-century Bruges master Hans Memling (ca. 1435-1494) is its finest yet -- for once the small, 20-odd pictures in the basement are every bit as compelling as the Frick’s own masterworks on the floor above.

Like many 15th-century painters, Memling left behind little information about himself. There are no letters or major contracts, and what information that can be gleaned from city and tax documents is dry and impersonal. We know that he was born in Seligenstadt in Germany, and after probably training in the workshop of Rogier van der Weyden in Brussels, he established himself in Bruges, then a bustling international city, by buying his citizenship in 1465. He soon became the town’s leading painter, was married with three children and owned a house in a prosperous section of town.

During his career he and his workshop produced dozens of private devotional pictures and several large altarpieces -- thanks to his exceptional productivity, there are more pictures by Memling than any other 15th-century Flemish painter. When I saw the giant Memling retrospective in Bruges in 1994, I realized that this was not without its consequences. Faced with so many blandly pretty Madonnas and stiffly drawn and rather empty altarpieces, one saw that here was a painter who, like his Italian contemporary Pietro Perugino, relied on a successful formula for most of his career. Although it's heretical to think so, I found that only rarely did Memling create a vibrant, haunting work of art -- among the exceptions were the Passion panel from Turin and The Seven Joys of Mary in Munich, the Crucifixion altarpiece from Lübeck, the Sacred Allegory from Strasbourg, the Shrine of St. Ursula, the overwhelming Last Judgment from Gdansk and the Lehman Annunciation -- and nearly all of his portraits.

In the 15th century, the independent portrait was something of a novelty. With the magnificent exceptions of Jan van Eyck and to a lesser extent those of Robert Campin and Rogier van der Weyden, flesh-and-blood people were most often portrayed in prayer, kneeling before the Madonna or a crucifixion in altarpieces, devotional diptychs or in miniature in an illuminated manuscript. While Memling produced a fair number of such "donor" portraits, it is his independent portraits that reveal his true originality. Most earlier 15th-century Flemish portraits depict the sitter on a plain, solid background (originally blues or greens which irreversible pigment aging over the last 500 years have darkened to black). Memling is one of the first painters to bring his subjects outdoors, setting them before sunny skies and placid verdant landscapes, occasionally framed by columns of colored marble. And while his Madonnas and saints have a cookie-cutter bland anonymity to them, Memling’s portraits are all sharply defined yet memorably mundane.

His sitters present themselves before us with little fuss and few accessories. Mostly a rather plain lot who never swagger and seldom look one in the eye, Memling’s subjects often have the slightly distant, blank gaze that one encounters in crowded subway cars. The painter doesn’t probe deeply, and has no psychological introspection. We’d never guess by looking at the pale and pious visage of the great clerical musician and composer Gilles Joye (one of the rare sitters who can be identified) that he was most appealingly lecherous and foul-mouthed, frequently reprimanded by his superiors for having a mistress and, most memorably, concocting obscene songs about all of his colleagues at Saint Donatian’s during a Christmas Day mass and singing them that night at dinner.

Though the Frick exhibition features several beautiful panels of Flemish subjects -- the thin-faced yet ruddy Man with a Red Hat from Frankfurt, and the reunited couple of An Elderly Man (from the Berlin Museum) and his modest and slightly weary wife (from the Louvre), it is Memling’s portraits of dark-eyed Italians, commissioned by visiting members and associates of Florentine banking houses and textile merchants, that are the exhibition’s high points. Once brought home, Memling’s Italian portraits, with their sunny vistas and forthright realism, revolutionized Italian painting. The catalogue essay by Paula Nuttall makes the excellent case that painters as various as Botticelli, Perugino, Leonardo and Raphael were affected by them. There is solid, irrefutable evidence for this: An Early Fra Bartolommeo portrait in the Metropolitan Museum seems to have cribbed its background from the newly cleaned Memling Portrait of a Man in a Fur-Spotted Collar in the Uffizi, and the column-bisected landscape from Memling’s Portrait of a Man in the Met’s Lehman wing was copied by Domenico Ghirlandaio in an early Madonna in the Louvre.

The exhibition concludes with one of the greatest masterpieces of 15th-century Flemish painting, Memling’s devotional diptych of Maarten van Nieuwenhove adoring the Virgin and Child, commissioned by the 23-year-old future burgomaster of Bruges. Still in its original hinged frame, the diptych is one of Memling’s most exquisitely finished and detailed works, set in an interior resplendent with stained glass windows depicting the sitter’s coat-of-arms and patron saint. There is perhaps no better example of Memling’s artistic variety, if not split personality. The Madonna and Child are delicately beautiful, refined yet oddly unreal and otherworldly, while young Martin, earnest, doughy and stolidly featured, is, despite the gap of 500 years, very much ours.

PAUL JEROMACK is a New York critic and journalist.