Little is known about the milk composition of nonhuman primates, and it has never been examined in capuchin monkeys (genus Cebus). This article reports on the macronutrient milk composition (fat, crude protein (CP), lactose, dry matter (DM), and total gross energy (GE)) of captive housed tufted capuchins (Cebus apella) (n=8). C. apella milk averaged 5.22% fat, 2.40% CP, 6.94% lactose, 16.48% DM, and 0.89 kcal/g. Fat was the most variable macronutrient and was significantly higher in samples collected after 2 months of lactation. To explore the adaptive significance of C. apella milk composition, results were compared with data on milk composition from a closely related cebid, Saimiri boliviensis boliviensis, and another large-brained anthropoid, Homo sapiens. C. apella milk was only significantly different from Saimiri milk in CP and the proportion of energy from CP. Compared with human milk, C. apella milk was lower in lactose but higher in fat, CP, DM, GE, and the proportion of energy from CP. Results from this small dataset suggest that among anthropoid primates, the macronutrient composition of milk is influenced by phylogeny, may vary relative to infant growth rates, but may not be related in any direct way to relative brain size. Am. J. Primatol. 72:81–86, 2010. © 2009 Wiley-Liss, Inc.