Infections are assumed to play a role in coronary artery disease (CAD) and cardiomyopathies. It is unknown whether the seroprevalence of antibodies to these microorganisms is higher in patients with than without CAD. The seroprevalence of antibodies to Bartonella henselae, Borrelia burgdorferi, Chlamydia pneumoniae, Coxiella burnetii, Helicobacter pylori, human granulocytic Ehrlichia, Leptospira, Rickettsia conorii, and Treponema pallidum was assessed prospectively in patients with exertional dyspnea or anginal chest pain who underwent coronary angiography because of suspected CAD. Patients with normal angiograms (NA) were those in whom no more than 50% stenosis of any coronary artery was found. Patients with CAD were patients who underwent percutaneous transluminal coronary angioplasty. There were 50 patients with CAD (9 female) and 62 with NA (25 female), with a mean age of 62 years. All patients had antibodies to at least one microorganism: to B. henselae, 8% of CAD patients and 5% of NA patients; to B. burgdorferi IgG, 14% CAD and 6% NA; to B. burgdorferi IgM, 6% CAD and 3% NA; to C. pneumoniae lipopolysaccharide (LPS) IgA, 76% CAD and 77% NA; to C. pneumoniae LPS IgG, 80% CAD and 90% NA; to C. burnetii, 0% CAD and 5% NA; to H. pylori, 92% CAD and 68% NA; to human granulocytic Ehrlichia, 8% CAD and 3% NA; to Leptospira IgG, 4% CAD and 2% NA; to R. conorii, 10% in both groups; and to T. pallidum, 2% CAD and 0% NA. The seroprevalence of antibodies to micro-organisms known to induce arterial and myocardial damage does not differ between patients with CAD and NA.