The World Health Organization recommends starting complementary feeding among infants beginning at 6 months old, as opposed to 4 to 6 months old. We prospectively studied 21 formula-fed infants beginning complementary feeding at 4 to 6 months old and compared them with 20 formula-fed infants starting feeding at 6 months old to determine difference in growth, number of infections and feeding habits. The studied infants were each enrolled at age 4 months. The decision as to which group the infants were classified into was based on the parental decision as to when to start complementary feeding. Initial demographic data were obtained for each subject. Growth, infections, and feeding habit data were recorded. No significant differences in growth were detected between the 2 groups. Respiratory infections at age 10 to 12 months were more common among children who began complementary feeding later. By age 12 months, the percentages of subjects who were bottle feeding and night feeding, and new food acceptance were not different from each other, but those who began complementary feeding at age 6 months were less picky eaters. By 15 months old, those who began complementary feeding at age 6 months had less bottle feeding and better food acceptance. In conclusion, for formula-fed infants, age of onset of complementary feeding was not associated with infant growth or infection rates. However, some feeding habits differed between the two groups. It is unclear if the age of introducing complementary feeding caused these differences or was merely associated with these differences.