Penile length, penile diameter and testicular volume at birth reflect the activity and normality of the hypothalamus-pituitary-testicular axis. In this prospective longitudinal study, we aimed to measure penis and testicular size in healthy newborns at birth and 6 and 12 months of age. Seven hundred forty-six term healthy male newborns were enrolled in the study. According to measurements taken within 48 hours after birth, mean penile length (MPL) was 2.81 +/- 0.32 cm, mean penile diameter (MPD) 1.04 +/- 0.09 cm, right testicular volume (TV) 1.73 +/- 0.45 ml, and left TV 1.64 +/- 0.48 ml. At 6 months of age, MPL was 3.67 +/- 0.35 cm, MPD 1.23 +/- 0.08 cm, right TV 2.07 +/- 0.31 ml, and left TV 2.00 +/- 0.27 ml. At 12 months of age, MPL was 4.47 +/- 0.43 cm, MPD 1.25 +/- 0.09 cm, right TV 2.01 +/- 0.12 ml, and left TV 2.01 +/- 0.13 ml. There was a weak albeit statistically significant correlation between weight, length and PL at birth. In conclusion, the MPL of newborns was shorter than that reported in the published data from other populations. This situation may be related to ethnicity in addition to the lower birth weight and length of our subjects when compared to the newborns in other populations. We observed a significant increase, more marked during the first six months, in MPL, MPD and TV, which can be explained by the hormonal mechanism during minipuberty.