The clinical features in 12 women with a circulating coagulation inhibitor (lupus 'anticoagulant') were studied and correlated with findings in 22 renal biopsies in these patients. Four of 12 women had serological evidence of systemic lupus. Only two of 23 pregnancies in these women were successful. Lesions in all biopsies were remarkably similar. Seven biopsies taken during or soon after pregnancy showed acute fibrinoid lesions. 'Acute' lesions consisted of fibrin thrombi in glomeruli arterioles and arteries and double contours in glomerular capillary walls. Biopsies carried out remote from pregnancy in these seven patients showed persisting double contours in glomeruli together with narrowing of arteries due to recanalizing thrombi and cellular intimal proliferation. All other biopsies showed similar lesions. The lesions were essentially the same as those of thrombotic microangiopathy and those described previously in post-partum renal failure. Renal function was severely impaired in four patients and showed moderate impairment in two patients at the time that acute thrombotic lesions were demonstrated in glomeruli. Renal function in two patients improved during plasma exchange therapy. Because all biopsies showed either acute, healing or healed, thrombotic lesions, it is proposed that these lesions are characteristic of the renal findings in patients with a lupus 'anticoagulant'. In all but two cases the acute lesions were seen during or soon after pregnancy.