The aim of the study was to investigate the long-term outcome of non-specific seronegative oligoarthritis in adults. The study included 64 adult patients with recent (<6 months) seronegative oligoarthritis (rheumatoid factor negative, number of swollen joints 1-4 during the first 6 months). Follow-up examinations were carried out at onset and at 1, 3 and 8 years from entry. A total of 47 patients attended the 23-year follow-up. The endpoint outcome was good. Seven had mild erosions in the hands or feet. Only one patient with HLA-B27 developed bilateral sacroiliitis. Three patients had retired from work because of joint disease. The functional outcome of the patients analysed by HAQ was very good after 23 years: 0 in 33 and 0.1-0.9 in 12 of the 47 patients. Reclassification revealed a certain heterogeneity: one case each of rheumatoid arthritis, SLE and ankylosing spondylitis, two cases of post-traumatic arthritis, four of osteoarthrosis, and six of possible reactive arthritis. Out of the remaining 49 patients 15 were HLA-B27 positive and 16 had at least one of the psoriasis-related HLA antigens (HLA-B13, 17, w16). In conclusion, our 23-year prospective follow-up study of patients with seronegative oligoarthritis confirms their favourable outcome. The reason is that the endpoint diagnoses seemed to be similar to those of mild spondylarthropathies.