OBJECTIVE To compare postnatal growth preceding the sudden infant death syndrome (SIDS) with that of age matched controls. METHODS Retrospective case-control study. Each SIDS victim was matched with two controls on date of parental interview, postnatal age, and neighborhood. Clinical and demographic data were collected by parental interview and by review of medical records, and interval body weights were obtained from health visitors' records. METHODS All infants dying of SIDS between 1 May, 1987 and 30 April, 1989 in a geographically defined region consisting of four health districts in Avon and North Somerset in southwest England. Seventy-eight of the 99 SIDS victims and 139 of 156 control infants were included in the final analysis. RESULTS There was no significant difference between SIDS victims and the controls in either of the two indices of postnatal growth which were analyzed. The mean growth rates (+/- 1 SEM) between birth and the last live weight (age equivalent weight for control infants) were 27.1 +/- 1.0 g/day for the SIDS cases and 28.3 +/- 1.5 g/day for the control infants. The mean growth rate (+/- 1 SEM) between the last two live weights were 31.5 +/- 2.9 and 24.9 +/- 2.1 g/day for the SIDS and control infants, respectively. Stratification of the infants by sex, gestational age, maternal smoking during pregnancy, breast versus bottle feeding, or age at death, did not result in any significant differences between SIDS and controls in either of the indices of postnatal growth rate. The 20 SIDS cases which were excluded from the final analysis did not differ from 78 whose data was analyzed, with regard to established SIDS risk factors, age at death, or postmortem weight. CONCLUSIONS No difference was found between the postnatal growth of SIDS victims and that of age matched control infants.