Up to 44% of patients with systemic lupus erythematosus have antiphospholipid antibodies, which in some patients have been associated with thrombocytopenia, in vitro coagulation abnormalities, and a thrombotic tendency. Patients with lupus nephritis who reach end-stage renal disease may continue to have anticardiolipin antibodies (ACL-Ab) at the time of renal transplantation despite a lack of clinical or serologic activity of systemic lupus. The risk of thrombotic and other complications has not been previously reported in these patients. To evaluate the effect of ACL-Ab on the course of renal transplantation, the clinical course of eight ACL-Ab-positive lupus patients transplanted in the period from January 1990 to June 1992 were compared with five ACL-Ab-negative lupus patients transplanted during the same period. All patients had lupus nephritis as the cause of renal failure. There were four thrombotic episodes in the patients who were ACL-Ab-positive but none in the control group. Neither of the two groups differed in the number of rejection episodes, the rate of graft loss, or renal function at last follow-up. Patients with systemic lupus erythematosus who are ACL-Ab-positive can be successfully transplanted. However, these patients require attention to possible thrombotic events with a potential role for prophylactic anticoagulation.