We examined platelet aggregation in platelet-rich plasma (PRP) and in whole blood in nine patients with Thrombasthenia Glanzmann (TG). In PRP, aggregation was measured by monitoring the changes in light absorbance that occurred in response to adenosine 5-diphosphate (ADP), collagen and ristocetin. To measure platelet aggregation in whole blood, we used multiple electrode Impedance aggregometry using the same aggregating agents. In PRP, the patient's platelets showed defective aggregation in response to ADP, collagen, epinephrine and partially to ristocetin in all patients. In whole blood, platelet aggregation in response to the same aggregating agents showed similar response and appeared to be very similar to that which occurred in PRP. Whole blood impedance aggregometry seems to give similar results to PRP light transmission aggregometry in patients with TG. Multiple electrode aggregometry (MEA) is faster and more convenient to use in these patients.