Exercise-induced collapse and sudden death are unusual in childhood. For this reason, a study was undertaken of a series of 12 cases of sudden death in childhood occurring during physical exertion associated with sporting activities. The age range was 7 to 16 years (mean 12.3 years, M:F ratio 5:1). Deaths resulted from trauma associated with the sporting activity, from an idiosyncratic response to exertion, or from exacerbation of a known underlying disease. Trauma was directly fatal (n = 4: vascular trauma in 1, head injury in 2, drowning in 1), exacerbated an underlying medical condition (n = 1: hypertrophic obstructive cardiomyopathy), or followed collapse from underlying organic disease (n = 1: drowning in epilepsy). Deaths after exertion occurred when there was an unexpected response to underlying occult disease (n = 4: aortic stenosis in 1, cerebral arteriovenous malformation in 1, hypertrophic obstructive cardiomyopathy in 1, coronary atherosclerosis in 1) or to preexisting known disease (n = 2: surgically corrected transposition of the great vessels in 1, asthma in 1). The fatal episodes often resulted from a complex interplay of a variety of factors, including physical exertion, possible trauma, and underlying organic disease. Testing of other family members may be indicated in cases where a rare, possibly familial, disease is found. Evaluation of cases required descriptions of activities before death, information from the medical history of the deceased, and detailed findings from the autopsy.