Plasma fibronectin was measured by ELISA in 25 samples from 22 patients with systemic lupus erythematosus (SLE). The mean fibronectin level for the entire patient group (654 micrograms/ml) was greater than that of normal controls (450 micrograms/ml), with highest levels observed in the subgroup of patients with severe disease activity (838 micrograms/ml) followed by those with moderate disease activity (732 micrograms/ml) (p = .04). Fourteen patients with other rheumatic disease had a mean fibronectin level of 407 micrograms/ml. Decreases in fibronectin levels corresponded to clinical improvement and reductions in DNA binding. Although elevated fibronectin levels did not correspond to any specific pattern of organ system involvement, fibronectin levels seem to parallel disease activity in certain patients. Future longitudinal studies of plasma fibronectin in SLE may further define its role as an indicator of disease activity.