BACKGROUND: Longitudinal data on bone age progression is scarce. AIM: The study aimed to present reference values for Tanner-Whitehouse 3 (TW3) bone age score and score increments, and to provide means and standard deviations of appearance time for all TW3 stages. Gender differences and differences between radio ulna and short bones (RUS) and carpal bone (CB) scores were studied. SUBJECTS AND METHODS: Bone age data collected for ages 3 months to 20 years in 232 subjects during the First Zurich Longitudinal Study (1954-1976) were used. Smoothed empirical percentiles of TW3 RUS and CB scores for age, of score increments for age and of score increments for attained score are presented. Means and standard deviations of the appearance times are calculated by parametric censored regression. RESULTS: There are clear differences between the RUS and CB scores and between the genders. Boys are delayed with respect to girls, with different delays for RUS and CB. For RUS, differences in maturation reflect the known differences of physical growth, with a later and more intense peak in boys. For CB, there is little difference in timing and intensity. However, girls reach the final score about 2 years earlier than boys. The consistently earlier mean appearance times in girls indicate that skeletal maturation is, already in childhood, more rapid in girls than in boys. There are significant gender differences in the sequence of appearance. CONCLUSION: Reference values for TW3 score and score increments and mean appearance times for stages add to existing knowledge and indicate important RUS/CB and gender differences, whose sources are largely unknown.