Cardiac involvement is a rare complication of infection by the influenza B virus. It usually presents with ventricular dysfunction, arrhythmias, or both. We report a 13-year-old boy with clinical, electrocardiographic, and laboratory findings of myocardial ischaemia during an otherwise silent acute infection with influenza B. Coronary endothelial injury constituted a potential underlying mechanism, and microthrombosis was promoted by high levels of lipoprotein(a) in the serum.