The primary antiphospholipid/anticardiolipin syndrome is a recently described entity wherein multiorgan thrombotic events occur in the absence of objective evidence of systemic lupus erythematosus. The spectrum of renal involvement remains poorly described. Two patients with coagulation abnormalities consistent with the primary antiphospholipid/anticardiolipin syndrome who developed profound renal insufficiency are reported. Striking microangiopathic lesions were documented on renal biopsy. Renal function improved concomitant to the institution of steroid therapy. Reversible renal failure should be added to the spectrum of clinical manifestations of this entity. The diagnosis of the primary antiphospholipid/anticardiolipin syndrome should be contemplated in individuals with unexplained acute renal failure.