In theory, sunshine exposure is sufficient to maintain normal vitamin D concentrations for the optimal growth of newborn infants. To determine whether season of birth, latitude (north v. south) and increasing dosages of vitamin D supplements would influence the growth rate for the first 6 months of life, 255 healthy fall-and spring-born infants from two northern and two southern cities in China were randomly assigned to receive either 100, 200, or 400 IU of vitamin D a day. The study showed that season of birth and dose of vitamin D did not affect the growth rate of infants born in the same latitude, but a significant difference was found in the gain in length over the 6-month period between infants from the north and infants from the south (P = 0.0001). Regional differences among the Chinese people, other than sunshine exposure, may have influenced the difference in length gain.