It has recently been suggested that a subpopulation of patients with rheumatoid arthritis, diagnosed on clinical, radiologic and pragmatic grounds, but with negative rheumatoid factor tests, represents a clinical entity quite distinct from that of seropositive rheumatoid arthritis. We have studied 60 sequentially presenting patients, 30 of whom were selected because they were seronegative, and 30 selected because they were seropositive in regard to IGM rheumatoid factor. The only major differences detected between the two groups on "blind' assessment were a greater tendency to deformity, a greater degree of erosion and the presence of subcutaneous nodules in the seropositive group. Seronegative and seropositive rheumatoid arthritis appear to have very similar clinical features, but differing degrees of severity.