The association of both viral and bacterial infections with acute myocardial infarction was investigated in a case-control study involving 40 consecutive patients with an acute myocardial infarction, 41 random controls and 30 patients with chronic coronary heart disease. All individuals were males aged 50 years or less. A rise in enterobacterial common antigen antibodies (15/40) and a recent influenza-like illness (11/40) were significantly more common among patients with acute myocardial infarction compared with the other groups. No differences were observed between the groups in the occurrence of antibodies against eight other bacterial antigens or 16 viruses.