Abstract Bone age of 335 newborns from Southern Iran where the average birth weight is lower than in Western countries (Sarram and Saadatnejadi, 1967) was investigated by radiologic evaluation of the knee joint ossification centers. No ossification center could be detected before the 36th week of gestation. Distal femoral and proximal tibial ossification centers appear from the 36th week of pregnancy with higher incidence for distal femoral center. The more advanced the pregnancy the higher the chance of knee joint ossification centers being present. Bone age is more advanced in female than in male infants and is delayed in small-for-date babies.