OBJECTIVE: The objective of our study was to determine the prevalence, distribution, and thickness of physiologic subperiosteal new bone formation in neonates and infants. MATERIALS AND METHODS: High-detail postmortem skeletal radiologic surveys of 101 neonates and infants who had died from sudden infant death syndrome were reviewed. The average age at the time of death was 2.6 months (range, 2 weeks-8 months 2 weeks). The location, distribution, and thickness of subperiosteal new bone formation of the long bones were noted and measured with an ocular magnification system. RESULTS: subperiosteal new bone formation was identified in 35 infants (35%), all of whom were between 1 and 4 months of age. The prevalence of subperiosteal new bone formation involving one or more bones was 37% for ages 1-2 months, 55% for ages 2-3 months, and 35% for ages 3-4 months. subperiosteal new bone formation involved the tibia in 29 infants (bilateral involvement in 19) and the femurs in 14 infants (bilateral involvement in 11). The humerus was a site of subperiosteal new bone formation in 12 infants; the ulna, in seven infants; and the radius, in two infants. The thickness of the subperiosteal new bone formation was never more than 1.8 mm, and the average thickness for the various long bones ranged from 0.7 to 0.9 mm. CONCLUSION: Our results suggest that subperiosteal new bone formation is a common finding in infants 1-4 months of age. It is potentially an abnormal finding in neonates (CI = 0.0, 0.007) and in infants older than 4 months of age (CI = 0.0, 0.003). A thickness of 2 mm or more in subperiosteal new bone formation is likely to be abnormal and should prompt further diagnostic evaluation.