We report a 9-year-old, previously healthy girl who died suddenly and unexpectedly and was found at postmortem examination to have a cardiac rhabdomyoma, megalencephaly, and an involuting adrenal ganglioneuroma. Her death was possibly caused by a fatal cardiac arrhythmia resulting from interference of the ventricular septal rhabdomyoma with the cardiac conduction fibers. Her extended family history included a variety of disorders, including cleft lip and palate and ill-defined cardiac and neurologic diseases. The constellation of her autopsy findings suggested a diagnosis of tuberous sclerosis, for which there are gene defects that can be identified in surviving family members.