The purpose of this investigation was to demonstrate platelet aggregation defects in patients infested with Plasmodium, giant and normal platelets counts. Laboratory studies were performed by using a “Payton Aggregometer”. ADP, Collagen, Epinephrine and Ristocetin as commercial reagents. Coagulation tests were done by regular methods. Thirty out of 31 patients were found to have marked defects in secondary aggregation wave with ADP. Aggregation with Collagen was constantly abnormal. Fifteen patients showed primary and secondary aggregation abnormality with ADP, Adrenaline and Collagen. Aggregation with Ristocetin was always normal. Aggregation defects were not drug induced since the patients were taking no medicines at least five days prior to studies. Studies done in volunteers taking antimalarics, Dipirone, Acetaminophen showed no interference with aggregation. Aggregation abnormalities were mostly found in patients infested with chronic Falciparum malaria. Most of the patients did not have bleeding manifestations but had multiple organ involvement. We think this platelet defect is a manifestation of some defense mechanism to prevent massive thrombosis frequently found in Falciparum Malaria.