The role of arachidonic acid in patients with acute myocardial infarction (AMI) was investigated. Plasma levels of thromboxane (TX) B2, 6-keto-prostaglandin F1 alpha (6KPG), leukotriene (LT) B4, LTC4, peptideLTs (LTC4 + LTD4 + LTE4) were evaluated in 19 AMI patients, 2 unstable angina (UA) patients and 12 normal controls. Blood samples were obtained from the femoral and pulmonary arteries. Further, plasma levels of LTC4 from the antecubital vein were determined in 14 AMI patients on admission and 10 healthy volunteers. Plasma levels of TXB2, LTB4, LTC4, peptideLTs from arterial blood significantly increased in AMI patients at the acute stage and one day after an attack of AMI compared with those of normal controls. TXB2, LTB4, LTC4, peptideLTs levels in UA patients were higher than those of normal controls. TXB2, LTC4 and peptideLTs levels on week after an attack of AMI significantly decreased compared with those at the acute stage, and these values decreased to normal control levels one month after an attack of AMI. LTB4 levels did not decrease one week after an attack of AMI but they significantly decreased one month after an attack compared with those at the acute stage. These values from the pulmonary artery were not significantly different from the values from the femoral artery obtained at the equivalent phases. Plasma levels of 6KPG did not show significant serial changes either. But the patients showing high levels of 6KPG in the acute stage showed lower levels of CPK and lower Wanger's QRS score than the patients showing low levels of 6KPG. Plasma LTC4 levels in the venous blood of 14 AMI patients were also significantly higher than those of healthy volunteers. Serum levels of lipid peroxide, alpha-tocopherol and superoxide dismutase activity were also measured but did not show significant serial changes in AMI patients. These results in AMI patients suggest that LTB4, LTC4 and peptideLTs, which are lipoxygenase metabolites of arachidonic acid, play an important role in the pathogenesis of AMI attacks as well as TXB2 and 6KPG, which are cyclooxygenase metabolites of arachidonic acid.