PURPOSE: To assess the difference in the appearance of the proximal humeral epiphyseal ossification center, as seen on chest radiograph, between preterm and full-term infants at the same corrected ages. MATERIALS AND METHODS: Forty two preterm infants born at 26-35 weeks of gestational age and 218 normal full-term infants born at 38-42 weeks were investigated. Because of various perinatal problems, the preterm infants were treated at a neonatal intensive care unit. Proximal humeral epiphyseal ossification centers were evaluated from chest radiographs, and in cases of preterm infants, the corected age of 0 month was defined as postconceptional 40 weeks. In preterm infants, the numbers of chest radiographs obtained were 42 at 0 month, 40 at 1 month, 37 at 2 months and 36 at 3 months of corrected age, while in those who were full-term, the numbers were 103 cases at 0 month, 42 at 1 month,42 at 2 months and 31 at 3 months of age. In the preterm group, alkaline phosphatase, calcium, phosphorus levels and simple wrist radiographs were checked. We then evaluated the difference of appearance of the proximal humeral epiphyseal ossification center between preterm and full-term infants at the same corrected ages, as well as the difference between causative diseases, between the normal and abnormal serologic group and between the normal and abnormal wrist group in preterm infants at the same corrected ages. Using Fisher's exact test, the data were analysed. RESULTS: The incidences of the proximal humeral epiphyseal ossification center in preterm infants were 2.4% (1/42) at 0 month, 20.0% (8/40) at 1 month, 43.2% (16/37) at 2 months and 69.4% (25/36) at 3 months; infull-term infants, the figures were 24.3% (25/103) at 0 month, 66.7% (28/42) at 1 month, 83.3% (35/42) at 2 months and 90.3% (28/31) at 3 months. At 0, 1, and 2 months, the incidences were thus seen to be lower in preterm than infull-term infants(p<0.001), though at 3 months there was no difference(p>0.05). In preterm infants, there were no statistical differences between causative diseases, between the normal and abnormal serologic group and between the normal and abnormal wrist group(p>0.05). CONCLUSION: Up to the age of two months, the proximal humeral epiphyseal ossification center of preterm infants appears later than that of full-term infants at the same corrected age.