Nine azotemic patients who developed a coagulopathy associated with the use of either cephalosporin or moxalactam antibiotics are reported. The acute renal failure patients had neoplastic disorders and were considered to be septic at the time that multiple antibiotics were administered. Four of 5 chronic hemo- or peritoneal dialysis patients also received multiple antibiotics. Nevertheless, the coagulopathy seemed to be most closely associated with the administration of the cephalosporin. One patient received moxalactam as part of the combination therapy for diffuse pulmonary infiltration during renal transplant rejection. Bleeding occurred into the gastrointestinal tract in four patients, into the kidney-urinary tract in three patients, into vascular surgical sites in two patients, and one each into the pulmonary-bronchial and cerebral-ventricular systems. Five operations were performed in four patients: a nephrectomy for massive subcapsular hemorrhage with a prothrombin time that exceeded 100 seconds; arteriovenous graft complicated by post-operative bleeding associated with prolongation of the prothrombin time; elective femoral-popliteal bypass complicated by a prolonged prothrombin time, bleeding into the graft site, hypotension, and a subendocardial myocardial infarction; elective cholecystectomy complicated by a two unit bleed associated with a slightly prolonged prothrombin time, followed by elective femoral-popliteal bypass complicated by a fatal intercerebral bleed associated with a more than twice normal prothrombin time. Cephalosporins are most likely associated with Vitamin K deficiency. Moxalactam is more likely to be associated with platelet dysfunction. Monitoring of the prothrombin time for cephalosporins or the bleeding time for moxalactam is the most reliable way to prevent what may be rapid emergence of clinical bleeding in patients with renal failure.