Autopsy reports from 208 cases of sudden infant death syndrome (SIDS) were reviewed and divided into three groups: group 1, term infants without infection; group 2, term infants with mild infection, and group 3, premature infants. The brain weights of these children were matched by age and body length, separately for males and females, with reference values and a group of 53 children born at term who died from conditions that did not affect the central nervous system. The brains of children in groups 1 and 2 were heavier than reference values when matched by age and body length. The differences between group 1 and reference values tended to increase with age; however, when matched by body length, this tendency was no longer observed. Body lengths of SIDS cases at death were also greater than reference values. There was no difference between brain weights of children in groups 1 and 2. These data suggest that the enlarged brains were present from birth rather than the result of disproportionate postnatal growth. The cause of increased brain size in SIDS cases is unknown, as is its possible role in the events leading to sudden death.