Our objective was to define the renal involvement in primary antiphospholipid syndrome (APS). We studied 20 patients with primary APS. Fourteen were women, mean age 34.4 years. None met ARA criteria for systemic lupus erythematosus. All patients underwent complete renal function studies. The presence of hypertension was also investigated. Renal disease was found in 5 patients, and was characterized by proteinuria, hypertension and renal failure. Kidney biopsy was performed in these 5 patients, showing thromboses of the microvasculature, mesangiolysis, mesangial interposition, electron lucent subendothelial material and ischemic obsolescence of glomeruli. Arterioles showed luminal narrowing due to medial hypertrophy, mucoid thickening of the intima, thrombosis and fibrosis. We found renal disease in 25% of our patients with primary APS. Biopsy findings were consistent with a thrombotic microangiopathy involving both arterioles and glomerular capillaries.