We studied positivity for anti-cardiolipin antibody, intraglomerular capillary thrombi on renal biopsy, and the progression of renal disease in 51 patients (10 male and 41 female), mean age 37 years (range 17-65 years), with a diagnosis of systemic lupus erythematosis and clinically evident nephritis confirmed by renal biopsy. Serum creatinine, serum indicators of disease activity and biopsies were analysed in subgroups according to thrombi and anticardiolipin status. End-points were death or chronic dialysis requirement and survival. Degree of sclerosis, crescent formation and necrosed glomeruli were all greater in those specimens positive for thrombi and in those specimens of patients who were serum ACA-positive, suggesting a relationship to disease activity/severity at presentation. The increase in serum anti-DNA antibodies and ANA and the reduction in C3 and C4 were significant in ACA-positive patients, with a strong relationship to disease activity when compared with changes in the ACA-negative patients (p < 0.05 in all cases). There was no significant difference when patients were separated according to the presence or absence of thrombi. Renal function at presentation was worse in patients with intracapillary thrombi and ACA positivity (p = 0.085 and p = 0.042, respectively). All patients progressed, but only those with intracapillary thrombi or anti-cardiolipin antibody positivity had a significant deterioration in renal function. Twenty-one thrombotic episodes occurred in 14 patients, of whom 13 were ACA-positive. Anticardiolipin antibody is a strong predictor of the presence of intraglomerular thrombi in SLE patients with renal involvement. The presence of thrombi and/or anticardiolipin antibodies indicate a worse long-term renal outcome. Anti-cardiolipin antibody positivity is a strong predictor of systemic vascular thrombotic complications.