Modified Ivy bleeding time (template) and platelet aggregation to ADP, epinephrine, and collagen were studied in 26 uremic patients who had not recently ingested anti-platelet drugs. Regardless of the aggregating agent used, the abnormalities in platelet aggregation were often mild, even with advanced uremia, and frequently less severe than the effects of common anti-platelet drugs. The inhibition of collagen-induced aggregation was significantly correlated with both increased bleeding time and blood urea nitrogen. Platelet aggregation was not discriminative between clinically bleeding and non-bleeding groups of patients, but the bleeding time was helpful in this regard. In certain cases, the aggregometric patterns differed between drug-induced and uremic thrombocytopathies. Platelet aggregometry appears to be of little help clinically in assessing the severity of the uremic bleeding diathesis.