There is a growing body of evidence that Sudden Infant Death Syndrome (SIDS) victims are not completely normal and healthy, as was once believed. A variety of new information from several disciplines strongly suggests that the infant who dies suddenly and unexpectedly may do so because of subtle developmental, neurologic, cardiorespiratory, and metabolic defects that converge at a particularly vulnerable time. Associated factors may include the anatomic structure of the upper airway, upper respiratory infection, and stress-producing environmental conditions. This report on SIDS first presents the historical perspective, the epidemiology, and the etiology of the phenomenon and describes the research efforts of the National Institute of Child Health and Human Development. The second section outlines the current stage of knowledge about SIDS including recent pathologic findings and findings on the role of sleep. In the final section, recommendations are made for research in the areas of epidemiology, infection and immunology, metabolic factors, upper and lower airways, behavioral considerations, sleep, biochemical factors, and the central nervous system. A bibliography of 77 references is included. (JD)