A total of 146 consecutive patients between 18 and 60 yr of age with oligoarthritis of unknown origin (< or = 6 active joints, < or = 8 weeks duration) were examined by a variety of clinical, laboratory and microbiological investigations, and followed longitudinally for 24 weeks. Reactive arthritis was diagnosed in 46 patients (19 induced by Chlamydia trachomatis, 27 by enterobacteria), 62 had undifferentiated arthritis, eight other inflammatory arthritic diseases, 15 acute sarcoid arthritis and 15 non-inflammatory joint diseases. Group differences were found for many baseline variables, but with considerable overlap between the groups. A set of four clinical and laboratory variables (elevated CRP, genitourinary symptoms, metatarsophalangeal joint involvement. HLA B27) could predict reactive arthritis with a sensitivity of 69.2% and a specificity of 93.5%. A wide range of clinical and laboratory examinations are required to determine the final diagnosis in oligoarthritis, but individual and sets of clinical and laboratory measures may give helpful clues for the correct diagnosis.