PURPOSE: To ascertain the normal maturation of proximal epiphyseal ossification centers by monthly age duringthe first two years of life. MATERIALS AND METHODS: The distribution of age was 0 to 24 months. Six hundred andseventy-five infants were male and 436 were female;their ages were measured in months, and there was no evidenceof developmental problems. Proximal humeral epiphyseal ossification centers were evaluated from chestradiographs;if not visualized, they were graded as 0, and otherwise, as follows:Grade 1:visualized with poormargin or a diameter of less than 1/4 of metaphyseal width; Grade 2: visualized with good margin or a diameter ofmore than 1/4 of metaphyseal width;in grades 3 and 4, two ossification centers were visualized. Grade 3 indicatedthat one ossification center had the morphology of grade 2 and the other of grade 1, while in grade 4, bothossification centers were morphologically grade 2. Grade 5 indicated that two ossification centers were fused. Wethen assessed the relationship between the development of an ossification center and monthly age. RESULTS: Inmale children, modes were as follows:0 month, 0;1 month, 0 and 1;2-4 months, 1;5 months, 2;6 months, 1; 7-12months, 2;13-16 months, 3;17 months, 4;18 months, 2;19 months, 3 and 4;20-21 months, 4;22 months, 2 and 3; and23-24 months, 4. In female children, they were as follows:0 month, 0; 1-3 months, 1; 4 months, 2;5 months, 3;6months, 1; 7 months, 4;8 months, 3;9 months, 3 and 4;10 months, 4;11 months, 3;and 12-24 months, 4. In malechildren, mean ages were as follows:grade 0, 0.7 months; grade 1, 3.5 months;grade 2, 10.2 months;grade 3, 14.4months; grade 4, 18.1 months; and grade 5, 24 months. In female children, these means were as follows: grade 0,0.3 months; grade 1, 2.7 months; grade 2, 7 months; grade 3, 10.1 months; grade 4, 15 months; and grade 5, 20.8months. CONCLUSION: A proximal humeral epiphyseal ossification center shows regular maturational featuresaccording to monthly age.