During an eight-month study, acute serum samples were obtained from all 228 patients admitted with chest pain to a coronary-care unit. On admission a history of a recent influenza-like illness was given by the same proportion (5%) of those subsequently shown to have myocardial infarction, angina, or miscellaneous diagnoses. Myocardial infarction was diagnosed in 105 patients and serum samples were obtained from all of the 93 survivors during convalescence. Sera were also obtained from 99 age and sex matched controls from the same geographical area. Sera from the cases and controls were tested for Coxsackie B antibodies by microneutralisation. In 3 patients with MI and 2 controls significant increases in antibody titre occurred which indicated recent infection. The distribution of type-specific antibodies, geometric mean titres, seropositivity rates, and the prevalence of raised antibody titres were all identical in the cases and the controls. These results do not accord with observations in uncontrolled series which suggested a causal relation between infections with Coxsackie B viruses and myocardial infarction.