Measurement of the Southwick's anteroposterior (AP) angle (shaft epiphysis proximal femoral AP angle) is not only a useful tool for planning the surgical treatment of deformities caused by slipped capital femoral epiphysis, but seems to be also important for recognizing the risk of epiphysiolysis development in obese patients (increased AP angle) or to confirm the diagnosis of slipped capital femoral epiphysis (decreased AP angle). To establish normal reference values of the Southwick's AP angle, we studied 97 normal nonobese adolescents (42 females, 55 males), with ages ranging between 8 and 16 years. The mean (SD) AP angle was 151.2 (5.0), ranging from 140 to 164. The limits for the first (p25) and third (p75) quartiles were 148 and 155, respectively. No difference was observed in the AP angle in males when compared with females. The AP angle was evaluated according to sex, chronological age, bone age, weight, height, and pubertal stage of development. We observed an inverse correlation of the AP angle with chronological age (r=-0.57) and bone age (r=-0.52). A weak inverse correlation was also found with stature (r=-0.33). Only a tendency toward an inverse correlation with weight (r=-0.27) or body mass index (r=-0.26) was observed. No significant correlation with the pubertal stage was found. When chronological and bone ages were divided into intervals, a significant reduction of the AP angle was observed only in patients older than 14 years compared with those younger than 10 years of age. In this study, we propose that the AP angle should be considered to be normal if it varies between 148 and 155. We conclude that the normal AP angle does not depend on sex; however, it tends to decrease with stature, and chronological and bone ages. In the normal weight range also, the AP angle decreases, contrasting with our previous findings in obese adolescents, in which the AP angle increases with the severity of obesity.