Fibromuscular dysplasia is a non-atherosclerotic, non-inflammatory vascular disease that is most commonly found in the renal arteries of pre-menopausal women. However, this disease entity has rarely been described in the coronary arteries and small cardiac conduction arteries. To our knowledge, there is no recorded case of fibromuscular dysplasia in small cardiac conduction arteries in infants/children. In this report, we present a case of unexplained death of a 14 month old wherein routine histologic examination failed to reveal a cause of death, but subsequent submission of the cardiac conduction system (atrioventricular [AV] and sinoatrial [SA] node regions) demonstrated the presence of isolated AV nodal artery fibromuscular dysplasia, which was considered the cause of death. The case serves to emphasize the potential importance of evaluating the cardiac conduction system in cases where routine gross and microscopic autopsy fails to reveal an adequate explanation for death.