The hypothesis that preweaning nutrition influences adult fat cell number and adiposity was tested in baboons. Newborn baboons were fed Similac formulas with caloric densities of 40.5 kcal (underfed), 67.5 kcal (fed normally), and 94.5 kcal (overfed) per 100 g formula. From weaning (16 wk) until necropsy at 5 yr of age all baboons were fed the same diet. At necropsy, fat cell number and fat cell size in 10 fat depots were measured. Female baboons overfed as infants had markedly greater fat depot mass, primarily because of fat cell hypertrophy, than normally fed or underfed females. Overfed male baboons had a greater fat mass in 4 of 10 depots compared with males underfed or fed normally as infants. Underfeeding did not affect body weight, nor adipose mass of either sex. The results show that infant food intake does not have a major influence on the fat cell number of young adult baboons.