Occasionally patients with overlapping features of rheumatoid arthritis (RA) and systemic lupus erythematosus (SLE), termed "rhupus," have been encountered. We wanted to ascertain the frequency of such patients and determine whether they represent a unique overlap syndrome. Of approximately 7000 new patients evaluated over 11 years, we identified six patients who had, on the average, 6.7 American Rheumatism Association criteria for RA and 4.2 criteria for SLE. Criteria for RA included chronic symmetric arthritis with morning stiffness (six patients); subcutaneous nodules (two patients); positive rheumatoid factors test (four patients); and radiologic erosions (four patients). The criteria for SLE included malar rash (three patients); discoid lupus erythematosus (two patients); biopsy-proved nephritis (one patient); photosensitivity (one patient); leukopenia/thrombocytopenia (four patients); positive antinuclear antibodies or lupus erythematosus cell test (six patients); hypocomplementemia (two patients); and abnormal results from skin biopsy (two patients). During observations of up to ten years, the conditions of three patients were stable or improved, one died, and two were unavailable for follow-up. Patients usually did not have conditions that evolved to classic rheumatic disease patterns. Rhupus was not common and did not occur more frequently (0.09% prevalence among our patients) than expected from chance concurrence of SLE and RA (calculated at 1.2%). These observations confirm that rhupus indeed exists as a syndrome manifested by patients sharing features of probable coincidental concurrence of RA and SLE, but not as a unique clinical pathologic or immunologic syndrome. Appreciation of these patients with rhupus is important since their therapy and outcome differ from those having RA or SLE alone.