Authors have kept under review two groups of healthy full-term infants born from undisturbed pregnancy, who lived in good social conditions from birth to one year of age. One of the groups was given 500IU/die and the other 1200 IU/die vitamin D. Authors evaluated the effect of the different doses of vitamin D on the prophylaxis of the rickets. 100 infants were reviewed in both groups. The clinical, laboratory and radiological examination revealed symptoms referring to rickets at the half of infants in both groups as early as 3 month of age. In the group of infants having been given low prophylactic dose of vitamin D there was significantly larger number of patients showing clinical evidence for rickets in both ages. Also significant differences were found at the age of one year in the size of the anterior fontanelles and the head circumferences, the developing of dentition and walk and the disturbed rank of eruption of teeth.