Erythrocyte complement receptor type 1 (CR1) was measured in 71 patients with systemic lupus erythematosus (SLE) and 43 healthy controls. The level of erythrocyte CR1 in patients with SLE was significantly lower than that of controls and correlated with the disease activity of SLE. The more active the disease, the greater the decrease in erythrocyte CR1. The level of erythrocyte CR1 was inversely correlated with the level of circulating immune complexes (CIC) and was positively correlated with C3, CH50, factor B, and factor H. We also found lupus nephritis more frequently in patients with decreased erythrocyte CR1 than in those with normal levels of erythrocyte CR1. Our results suggest that the deficiency of erythrocyte CR1 in patients with SLE is acquired and may serve as a variable of clinical disease activity and may play a role in the pathogenesis of SLE.