Forty-one of 858 patients with systemic lupus erythematosus (SLE) developed clinical deformity of their hands. This deformity was clinically and radiologically different from that found in 40 patients with classical or definite rheumatoid arthritis (RA), and tended to appear early in the course of disease. Characteristics of this arthropathy included nonerosive carpal collapse; exceptional erosion of the styloid processes; Z deformity of the thumb; nonerosive ulnar deviation and subluxation of MCP joints; parametacarpophalangeal joint hook formation; scant and asymmetric joint erosions; and swan neck deformity of the fingers. Most of these changes seemed to be due to involvement of the ligaments rather than to the destructive effect of synovitis. Patients with SLE with deforming arthropathy had a higher frequency of rheumatoid factor positivity, sicca symptoms and antibodies to native DNA, whereas they had lower incidence of facial rash and photosensitivity than did those without. Other manifestations did not differ. We propose that most patients with SLE with deforming arthropathy belong to a subset of SLE rather than representing the coexistence of SLE and RA.