To determine the clinical evolution of Lyme arthritis, 55 patients who did not receive antibiotic therapy for erythema chronicum migrans were followed longitudinally for a mean duration of 6 years. Of the 55 patients, 11 (20%) had no subsequent manifestations of Lyme disease. From 1 day to 8 weeks after disease onset, 10 of the patients (18%) began to have brief episodes of joint, periarticular, or musculoskeletal pain for as long as 6 years, but they never developed objective joint abnormalities. From 4 days to 2 years after disease onset, 28 (51%) had one episode or began to have intermittent attacks of frank arthritis, primarily in large joints; a few had polyarticular movement. The total number of these patients who continued to have recurrences decreased by 10% to 20% each year. The remaining 6 patients (11%) developed chronic synovitis later in the illness; of these, 2 (4%) had erosions, and 1 (2%), permanent joint disability. The spectrum of Lyme arthritis ranges from subjective joint pain, to intermittent attacks of arthritis, to chronic erosive disease.