Deposits of hemosiderin are found frequently in the interstitium of the lungs of infants who die suddenly and have been a suggested marker for hypoxia during previous episodes of apnea. We studied the epidemiology of pulmonary interstitial hemosiderin (PIH) in 94 infants with a diagnosis of the sudden infant death syndrome that was established during the New Zealand Cot Death Study. Twenty-seven infants (29%) had widely distributed PIH. Associations were sought between PIH and variables on which information had been obtained from parental interviews or from medical records. Previous suggestions of associations with increasing birth weight and decreasing age at death were confirmed in multivariate analysis (P = 0.0018 and P = 0.03, respectively) and with increasing gestation in univariate analysis (P < 0.0001). PIH was not found in any of the 13 infants delivered by elective cesarean section compared with a 33% incidence in those exposed to labor (P = 0.02). There was no association with previous apnea. We consider that the likely explanation of these findings is that PIH is a residue of interstitial hemorrhage caused by chest compression during labor that occurs particularly in larger infants of greater gestational age and then clears gradually during early infancy.