We examined both clinically and by determining HLA-A, -B, -C and -DR antigens 50 patients thought to have seronegative erosive polyarticular rheumatoid arthritis (RA) in Finland and the USSR. All the patients fulfilled at least 4 of the 1987 ARA criteria for RA. According to HLA typing and clinical findings, of which a part was collected by followup, the patients fell into 5 groups: HLA-B27 related diseases, putative psoriatic arthritis, putative juvenile chronic polyarthritis, and seropositive and seronegative RA. Our results indicate that most of the patients with seronegative RA had some other disease. In the remaining cases the presence of rheumatoid factors had not been examined adequately, especially at the early phase of disease. The classification of erosive seronegative polyarticular patients is discussed.