One hundred twenty-five sudden infant death syndrome (SIDS) victims followed up since birth from a large prospective study were compared with matched controls. Some of the future SIDS victims showed evidences of neonatal brain dysfunction including abnormalities in respiration, feeding, temperature regulation, and specific neurologic tests. These abnormalities could not be ralated to events in labor or delivery. A greater proportion of the future victims were mildly underweight for gestational age. The gestations that produced the SIDS victims were characterized by a greater frequency of mothers who smoked cigarettes and had anemia. The demographic profile of SIDS families proved to be indentical to the profile for families with excessive perinatal mortality. Many of the SIDS victims showed a retardation in postnatal growth prior to death.