We describe a 31-year-old man with Down's syndrome who presented with severe chronic hypothyroidism and a massive pericardial effusion. Following partial aspiration of this effusion, he rapidly deteriorated and died. Findings at autopsy revealed him to have an atrioventricular septal defect with shunting at the atrial level. We postulate that, by releasing extrinsic pressure on his right heart by the aspiration, there was sudden shunting of blood from left-to-right, resulting in hypotension, shock, and subsequent death. We highlight the difficulties in management of such a case, and suggest alternative strategies.