The purpose of this study was to determine whether the use of relatively small amounts of radiographic contrast agents may impair hemostasis. Before and after injection of contrast agents, we measured the template bleeding time, platelet count in whole blood and platelet-rich plasma, platelet aggregation induced by epinephrine and adenosine diphosphate (ADP), and percent disaggregation of ADP-induced platelet aggregates in platelet-rich plasma of 18 patients. Ten patients received 50-100 ml of sodium iothalamate (66.8%) for intravenous urography and 8 patients received 125-200 ml of a combination of meglumine diatrizoate (66%) and sodium diatrizoate (10%) for coronary angiography and left ventriculography. Neither group demonstrated a statistically significant mean difference (p greater than 0.05) for any parameter measured before and after administration of the contrast agents. However, 2 patients who received diatrizoate had lengthening of their bleeding times to greater than twice the baseline value without any change in in vitro platelet aggregation. Physicians should be aware of this potential cause of bleeding which could be of particular danger to patients undergoing cardiopulmonary bypass with its inherent hemostatic defects.