Inflammation caused by activated macrophages and T lymphocytes may trigger plaque rapture in acute coronary syndrome (ACS). Anandamide and 2-arachidonylglycerol (2-AG) are macrophage-derived signal lipids and may be involved in the pathogenesis of ACS, but no clinical relevant data have been reported. In 43 acute myocardial infarction (AMI) patients (66 ± 2 years), blood samples were obtained from the aortic root and the infarct-related coronary artery (IRA) using a PercuSurge system during primary percutaneous coronary intervention (PCI). In six patients with stable effort angina (SEA) (56 ± 6 years), blood samples were obtained from the site of stenosis during elective PCI. In 25 of the 43 AMI patients, anandamide was detected in the serum. Serum anandamide level was 35 ± 20 pmol/mL in the aorta and was significantly increased to 401 ± 134 pmol/mL in the IRA (P < 0.01). 2-AG was undetectable in most of the patients. In patients with SEA, neither anandamide nor 2-AG was detected in the serum at the plaque site. In AMI patients with anandamide detected, left ventricular ejection fraction at 2 weeks after PCI was increased by 3.7 ± 2.1% compared with that at the acute phase, while it was decreased by 3.0 ± 1.8% in those without anandamide detected (P < 0.05). The serum anandamide level at the culprit lesion was elevated compared with the systemic level in a significant number of AMI patients, indicating the synthesis of anandamide at the IRA. Anandamide was suggested to be derived from ruptured plaque and may exert beneficial effects in humans.