Aims: This study was conducted prospectively to ascertain the prevalence of anticardiolipin antibodies (ACAs) in patients with lupus nephritis and to determine whether this subgroup of patients differed clinically and histologically from patients without the antibody. Patients and Methods: 40 SLE patients (26 Blacks, 14 Indians, 37 females, 3 males) with evidence of renal involvement underwent clinical assessment and percutaneous renal biopsy. Special investigations included: urinary protein quantitation; radioisotope glomerular filtration rate (GFR); complement levels, and antinuclear antibodies and ACAs. Histology was reviewed by a single senior pathologist blinded to the ACA results. In addition to the standard WHO classification, specimens were examined for intrarenal thrombosis. Results: The prevalence of ACA was 45% (18 of 40 patients). Thrombocytopenia was more frequent in patients with ACA (33 vs. 13.6%, p = 0.015). Patients with ACA did not differ from controls with regard to the incidence of thrombosis, neurological disorders, recurrent fetal loss, active disease and hypertension. Mean GFR and 24-hour urine protein (ACA vs. controls) were 51.3 versus 67 ml/min (NS) and 2.4 versus 3.7 g (NS), respectively. Intrarenal microvascular thrombosis (glomerular and arteriolar) occurred in 27.7% of ACA patients versus 9% of controls (p = 0.025). Apart from a higher incidence of class-III nephritis in the controls, standard histology (WHO classification) did not differ between the 2 groups. Conclusion: The prevalence of ACA in our patients with lupus nephritis was 45%. This subgroup did not differ from patients without the antibody apart from a higher incidence of thrombocytopenia and intrarenal microvascular thrombosis.