Neotropical primates show a remarkable range in body size, spanning two orders of magnitude from the tiny pygmy marmosets (100 g) to the woolly spider monkeys (11,000+ g). Even among the “smaller” platyrrhines, the range is large. In addition, these primates demonstrate a wide diversity in degrees and directions of sexual dimorphism, in both body size and canine size, from marked positive dimorphism (males larger than females), through monomorphic species, to negative dimorphism. Potential correlates or causes of the patterns of dimorphism in body size are investigated, including overall body size, natural selection for life history strategies, sexual selection, diet, habitat, and phylogenetic inertia. Focus is especially on those genera that show species-specific variation in dimorphism (e.g., Saguinus, Pithecia). Results are contrary to those for cross-primate or catarrhine studies, but complementary to recent studies on strepsirhines. They suggest that sexual selection is the primary determinant of degree and pattern of sexual dimorphism in platyrrhines, but that there is also a dietary effect. Natural selection may have some effect, although not the parameters analyzed here. Body size, habitat (primary vs. secondary forest preference), and phylogenetic inertia or constraints do not have any effect on the presence of sexual dimorphism in body weights in New World monkeys. © 1994 Wiley-Liss, Inc.