A group of twenty patients having proved heart-block without the customary causes such as coronary artery disease, digitalis, fever and rheumatic infection were studied to determine the incidence of diphtheria in childhood. The incidence was 50 per cent as compared to six per cent in 600 consecutive control surgical cases. The average age of patients with a positive diphtheria history was 11 years younger and their systolic blood pressure 40 mm. lower than was the case with those with a negative history. Diphtheria in childhood appears to be an etiological factor in the development of heart-block in later years.