LEUVEN, BELGIUM -- In a piece of detective work worthy of his fellow Belgian Hercule Poirot, Jan Dequeker, chief rheumatologist at the teaching hospital of Catholic University here, has used 15th-century Flemish art to effect a small revision of medical history.

In 1971, Dequeker was perusing a new textbook on rheumatology. The authors speculated that rheumatoid arthritis, the long-term affliction that causes swelling, pain and crippling in the joints, was a new disease that had not occurred before the early 19th century. There was a lack of evidence of the disease in medical writing, in literature, in works of art or paleontological investigation, the authors said, which bolstered their theory that rheumatoid arthritis was infectious, perhaps caused by a recent viral mutation.

Something about this troubled Dequeker, who works with patients suffering from various kinds of rheumatic diseases, including arthritis. For a reason he still can't explain, he thought there might be evidence of arthritic joints in old paintings.

The rheumatologist, long interested in art, had a collection of art catalogs at home, so more like Sherlock Holmes than Poirot, he got out his magnifying glass. He looked at pictures, concentrating on the hands, where symptoms of rheumatological disease usually appear first.

It didn't take long.

In a picture from the museum in his hometown, he discovered rheumatoid-like lesions on the hand of Christ in Jan Rombauts' "Christ Appearing to St. Peter," painted about 1500. "The hand looked like that of someone with longstanding rheumatoid arthritis," Dequeker said.

That same weekend, he found arthritis in the third joint of the left index finger in a famous portrait of Federigo de Montefeltre, by Joos van Gent. The painting hangs in the Ducal Palace in Urbino.

A drawing by Jan van Eyck that hangs in a Rotterdam museum showed that fingers on the right hand of John IV, Duke of Brabant and a founder of Leuven University, had the swan-neck curve, a symptom of rheumatoid arthritis.

But a picture by Jacob Jordaens in the Prado, which depicts Jordaens, his wife, child and housemaid, is one of Dequeker's best examples. "The hands of the housemaid are in disproportion with her nice young face," he said. "The second joints of the fingers and the knuckles are swollen. There's atrophy of the back of the hand, and the wrist is swollen. These are clinical signs from which you could make a diagnosis." In fact, Dequeker has a photograph of the hands of one of his patients with identical disfigurement.

"In a very short period, I found a number of examples," Dequeker said. He presented a paper on his findings in London in 1975, and the British Medical Journal published the paper in 1977. When the author of the 1971 textbook wrote a chapter on the history of arthritis for another manual, he mentioned Dequeker's work.

Dequeker warns that medical sleuths must not infer too much from the appearance of hands because painters often use them as hallmarks of their style. The Flemish painter Rogier van der Weyden, for instance, painted fingers that were particularly fine and long. Dequeker says the deformities he's talking about are not just stylistic aberrations.

Flemish painters, with their meticulous attention to every detail, are his best source. They portrayed people exactly as they were -- warts, swollen joints and all. Most of Dequeker's examples come from the period between 1400 and 1600. "Before that, the Greeks and Romans idealized people, and later the Baroque artists of the 17th and 18th centuries did as well," he said. "They weren't capable of such detailed accuracy."

A French doctor, A.J. Landre-Beauvais, made the first diagnosis of rheumatoid arthritis in 1800. Dequeker believes that early cases of the disease were not noted historically because life spans were too short for a sufficient number of cases to develop and be recognized as examples of a specific disease. "The composition of the population was completely different," he said. "The mean age was 30 to 40; infectious diseases wiped out the weak before they could get arthritis."

The origin of rheumatoid arthritis is still obscure. Dequeker thinks it is partly genetic, possibly involving the immune system, and that an environmental factor -- a virus or bacteria -- might play a part.

Dequeker has delivered papers on his unusual avocation all over the world. Each trip is an opportunity to visit another museum. "You have to look at thousands of paintings to find one good one," he said.

The work of Quinten Metsys, a painter who lived in Leuven from 1465 to 1530, has been a fertile source for Dequeker. In Metsys's "The Lamentation" in the National Museum in Lisbon, the fingertips of one of the four women show acrocyanosis, which means they turn blue with cold, and there's swelling in the back of the right hand. In Metsys's portrait of Erasmus, the first joint of the thumb is swollen and there is hyperextension of the thumb joint near the hand, meaning that it turns backward more than 180 degrees. Dequeker sees thickening at the joints of the second and third fingers of the right hand.

Viewing Sandro Botticelli's "The Birth of Venus" in the Uffizi Gallery in Florence, Dequeker saw that the fingers of her right hand were deformed by swollen joints. Her wrist was slightly swollen and the little finger twisted. Then he noticed that the hair locks followed the same pattern as the fingers of the right hand. Did Botticelli paint the fingers first, then the hair? Or the hair and then the fingers?

Was it mere coincidence?

When a doctor in Mexico said that he thought there was juvenile arthritis in the right hand of another Botticelli painting, "Portrait of a Youth," in the National Gallery in Washington, Dequeker went back to "The Birth of Venus." He decided the model did have rheumatoid arthritis. "On the left hand, the index finger was swollen," he said. "It was what we call a 'sausage finger' in arthritic patients." Historical sources reveal that the 16-year-old model, Simonetta Vespucci, died from tuberculosis two years after Botticelli did the painting. "It is not at all impossible that she suffered from reactive arthritis associated with tuberculosis," Dequeker said.

Very few scholars have written about medical problems in art. Dequeker found an article published in 1934, Mains pathologiques dans l'art (Pathological Hands in Art), which includes two paintings. In one by Marinus van Reymerswael (1497-1567) where St. Jerome meditates on death, the author saw characteristic signs of sclerosis, a disease of the skin. In another, a self-portrait by Michelangelo, he saw a swelling of the first and second joints of the fingers of the left hand, with atrophy of the muscles in the first and second fingers.

Around the turn of the century in France and Germany, there was some interest in the iconography of medicine. In 1903, Eugeen Hollander, a Berlin surgeon, published a book called "Medicine in Classic Paintings." None of these writers recognized rheumatoid arthritis in paintings, but they did describe neurological ailments and physical deformities they found in great works of art.

Although the Impressionists provide no fodder for Dequeker's search, the doctor has a special affection for Auguste Renoir. Because Renoir suffered from rheumatoid arthritis, Dequeker put the painter's self-portrait on the cover of his own textbook, "Rheumatism: A Diagnostic Problem."

Ann Waldron is a writer in Princeton, N.J.