A 4-year-old boy presented with a single seizure following a viral syndrome. He had a pericardial effusion on admission, and this increased suddenly on the third day of hospitalization, producing cardiac tamponade. After blood was aspirated from the child's pericardial cavity, the father revealed that he performed cardiac massage on his son following the seizure. A laceration of the right atrium was repaired at operation, and the boy made a good recovery. Cardiopulmonary resuscitation by lay persons is not without hazard, and patients with such a history should be watched carefully for the possibility of damage to intrathoracic structures.