Risk of sudden infant death syndrome (SIDS) reaches a maximum in the third month. Thereafter, it decreases by half every 40 days or so. It is proposed that the relative sparing of the very young infant is a consequence of an innate (but temporary) characteristic possessed by the newborn infant. Interpretation of available data suggests that this innate characteristic is negatively associated with the infant's level of maturity. This is the basis for the hypothesis that the age at which the risk of SIDS begins to decline at a uniform rate decreases as the infant's gestational age increases. Because of a greater level of maturity at birth, the age at which this occurs in the black infant should be earlier than average. An analysis of data on 32 573 instances of SIDS within the United States between 1985 and 1991 provides support for the hypothesis.