• Questions regarding fat deposition and hormonal control of lipogenesis in infants fed formulas containing medium-chain triglycerides have been raised by studies in animals. To compare growth, clinical observations, and laboratory measures of infants fed the same basic isocaloric formulas with 10%, 30%, or 50% of the total fat as medium-chain triglycerides, we studied longitudinally for the first four weeks of life 22 infants who weighed 1000 to 1750g. We found no differences in weight or skin-fold thickness between the groups. There were no clinical characteristics to distinguish one group from another. Two-hour postprandial levels of insulin and glucagon showed no differences one week to ten days after the start of the formula diet. The skin-fold thickness of all infants studied continued to increase after birth, regardless of weight loss. The increase in skin-fold thickness was comparable with the increase expected in the fetus in utero, but not significantly greater. The weight curve was parallel to the fetal weight curve but was lower, due to the first-week loss that possibly reflected a loss of body water, as suggested by the dynamic skin-fold thickness curve.