In this study we evaluated the effect of immunosuppressive drug, cyclophosphamide, on lymphocytes from systemic lupus erythematosus (SLE), using the micronucleus test. Micronucleated peripheral blood lymphocytes were analyzed in cytochalasin B treated binucleated lymphocytes. There was significant increase in cells with micronuclei (P < 0.001) in SLE patients who had received cyclophosphamide (10.05 +/- 4.7) in comparison to the patients with SLE who had not received any medical treatment (6.24 +/- 2.42) and control normal subjects (5.48 +/- 2.24). The SLE patients who used cyclophosphamide showed a slightly higher frequency of micronucleated cells in the first 48 h after treatment. The present study emphasized the lack of correlation between age, sex and frequency of micronuclei.