A prospective study of arthritis was performed in 47 patients with Behçet's disease followed up over a 47-month period (mean 19.25 months, SD 14.09). These patients had a total of 80 episodes of arthritis, which were analysed for joint distribution and symmetry, in 56 of which the duration could also be determined. Attacks were oligoarticular, affecting up to 4 joints per patient, 54 (68%) being monoarticular. Knees, ankles, and wrists were the most commonly involved joints. Involvement of spinal, shoulder, hip, and sacroiliac joints was rare. The arthritis was usually not deforming and subacute; 82% (46/56) of the attacks lasted for 2 months or less and 18% (10/56) for between 3 months and 4 years. The ESR was moderately elevated during the attacks. In 32 specimens the synovial fluid was inflammatory (cell count 14.7 +/- 10.1 x 10(9)/l), but in 19 (59%) a good mucin clot formed. Synovial biopsy in 12 patients revealed superficial ulceration, paucity of plasma cells, and in 5 instances lymphoid follicle formation.