131 cases of the Sudden Infant Death Syndrome (SIDS) among infants born in the Municipality of Copenhagen in 1956-71 were analysed on the basis of data collected prospectively by the infant health visitors. Compared with living controls, the frequency of maturity and the mean birth weight and length were lower among the SIDS infants. After making the SIDS and control groups comparable with respect to birth weight and type of feeding, we found a tendency towards a slower growth rate among the SIDS victims. The SIDS victims started to smile and prattle later than controls, while there was no difference as regards the average age at which the infants became able to hold their heads. It is concluded that the differences are so small and the individual variations so wide that birth weight and length, growth rate, and psychomotor development will not be useful predictors of the SIDS.