The objective of this study was to determine whether anticardiolipin antibodies (ACL) in SLE patients are associated with a specific pattern of lupus nephritis and/or with renal microvascular changes. Patients with SLE, followed prospectively between June 1991-May 1994 at The Wellesley Hospital Lupus Clinic, who underwent a renal biopsy were included. The ACL was measured by the ELISA according to international standardized method. Renal biopsy morphology was assessed using the WHO criteria for the classification of lupus nephritis. Renal vascular changes included glomerular hyaline thrombi, intimal fibrosis and intraluminal thrombi of renal arterioles. There were 23 SLE patients. The mean age at diagnosis of SLE was 28.2 years and the mean disease duration was 6.3 years. Of these 10 (43%) had high levels of ACL. No difference in the frequency of severe nephritis (Class III and IV) was identified amongst patients with and without ACL. Mesangial nephritis was more common in patients with ACL 40% vs 0, p = 0.02). Glomerular hyaline thrombi occurred in 3 (13%) patients. None of them had positive ACL. Renal vascular lesions included intimal proliferation in 4 (ACL + , 1) occluded lumens by thrombi in 2 (ACL + 1). Our data indicate that the development of glomerular and/or microvascular changes is not related to the presence of ACL.