The relative significance of traditional risk factors, chronic infections and autoimmune processes in the development of acute myocardial infarction (AMI) has not been fully elucidated. We compared serum IgG antibody titres to various pathogens, i.e. Chlamydia pneumoniae (Cpn), cytomegalovirus (CMV) and herpes simplex virus type 1 (HSV-1), and to the potential autoantigens human heat shock protein 60 (hHSP60) and mycobacterial heat shock protein 65 (mHSP65), in serum samples obtained from patients 3-48 h after AMI (n = 40) or stable effort angina (SEA, n = 43), and from controls (n = 46). The strongest association was observed between AMI and the elevated level of hHSP60 antibodies. The association between AMI and the level of Cpn antibodies was also significant. High levels of hHSP60 and Cpn antibodies represented independent risk factors for the development of AMI, but the simultaneous presence of high levels of antibodies to Cpn and hHSP60 suggested a joint effect on the relative risk of AMI (OR = 12.0-21.1). The antibody titres to mHSP65 were higher in the SEA group than in the controls, and the simultaneous presence of high levels of Cpn and mHSP65 antibodies meant an increased risk among the SEA patients. The antibody titres to CMV or HSV-1 were similar in the three groups. In conclusion, these results demonstrate associations of AMI with high levels of anti-hHSP60 and anti-Cpn antibodies, and of SEA with the level of anti-mHSP65 antibodies, these being independent risk factors.