Joint symptoms associated with B. burgdorferi infection range from arthralgias, to brief attacks of arthritis, to chronic erosive synovitis. From 2 weeks to 2 years after the onset of disease, commonly following migratory arthralgias, about 60% of untreated patients with Lyme disease in the United States develop brief attacks of oligoarticular arthritis, primarily in large joints, especially the knee. Episodes of arthritis often become longer during the second or third years of illness, lasting months rather than weeks, and in about 10% of patients, chronic arthritis begins during this period. Chronic Lyme arthritis appears to have an immunogentic basis. Of 28 patients with chronic arthritis, 25 (89%) had the HLA-DR4 or -DR2 specificities compared with only 6 of 22 patients (27%) with arthritis of short duration (P = 0.00006). In adults, Lyme arthritis is most like Reiter's syndrome or reactive arthritis, and in children, it is most similar to the pauciarticular form of juvenile rheumatoid arthritis.