Procoagulant alterations and thrombocytopenia in falciparum malaria correlate with parasitemia, serum levels of tumor necrosis factor alpha (TNF alpha), and clinical severity. Thus, heparin or acetylsalicylic acid (ASA), which are used frequently to prevent thrombosis and (in the case of ASA) to control fever, could be potentially beneficial. We randomized 97 patients with falciparum malaria into three groups: 33 patients received low-dose heparin subcutaneously, 31 received ASA intravenously, and 33 did not receive either drug. All patients received appropriate antiparasitic treatment. Eighteen of 97 patients (seven receiving heparin, five receiving ASA, and 6 in the control group) had complications upon admission. During therapy, elevated TNF alpha and lactate dehydrogenase levels and decreased platelet counts returned to normal values. Except for a minimal partial thromboplastin time prolongation with heparin, heparin or ASA did not affect any laboratory parameter, duration of parasitemia, fever clearance, or the length of hospitalization. Thus, it appears that ASA and heparin do not influence the course of falciparum malaria. Hence, in view of possible side effects, these substances should not be recommended for routine use in the treatment of human malaria.