Congenital cardiac diverticula are rare abnormalities that may occur as isolated malformations. In this report, we describe a case of an isolated congenital cardiac diverticulum complicated by a large serous pericardial effusion diagnosed ultrasonographically at 19 weeks' gestation. Therapeutic pericardiocentesis at 20 weeks' gestation resulted in complete resolution of the effusion with a normal fetal outcome. There is only one previous report of a prenatal diagnosis of a cardiac diverticulum complicated by a pericardial effusion and that patient underwent termination of pregnancy (Carles et al., 1995). Given the otherwise favourable prognosis for this lesion, and the excellent response in this case, pericardiocentesis should be considered in similar cases.