Quantitative analysis of reticular dendritic spines was performed on rapid Golgi impregnated neurons in 7 brainstem areas from 61 sudden infant death syndrome (SIDS) and 34 control infants. Throughout the first postnatal year, mean spine density in SIDS was significantly greater than the mean density in controls (P less than 0.0001). There were significantly higher values of spine density in SIDS compared to controls (P less than 0.0001) in both term and preterm infants. Within the SIDS brainstem itself, the density of dendritic spines was significantly different (P less than 0.05) between two medullary regions and between reticular and non-reticular formation areas. Among these brainstem areas in controls, there was no significant difference. Our findings indicate an immature developmental pattern of increased dendritic spine density in the SIDS brainstem which may be responsible for abnormal central respiratory and arousal control. These significant quantitative differences in spine density are considered in the present study to represent an anatomical substrate of brainstem immaturity in the multifactorial pathogenesis of SIDS.