Perhaps the most fruitful dividend from research into the sudden infant death syndrome (SIDS) in the past decade has been its establishment as a distinct disease entity. Though this knowledge is still not universally distributed throughout the medical profession, at least some of the veil of mystery and superstition has been lifted, and authoritative guidelines for the recognition and management of SIDS have been issued. Among the characteristics of SIDS, four deserve special emphasis, and serve as the basis for most of the research currently being conducted: (1) Most cases of SIDS occur between 4 and 16 weeks, suggesting some physiologic vulnerability during this age period.