Chylopericardium is one of the rarest complications of cardiac surgery. The authors report only the 19th case, that of a 12-year-old boy. The chylopericardium developed after an aortic valvotomy, even though the retropericardial portion of the mediastinum was entered only to pass a tape around the ascending aorta. Etiologic factors are directly or indirectly related to injury to the thoracic duct or tis tributaries. Treatment initially is conservative, consisting of adequate pericardial drainage and a low-fat, medium-chain triglyceride diet. If this fails, the appropriate operative procedure is carried out.