We present body mass (N = 419) and crown-rump length (CRL, N = 210) measurements from 38 male and 49 female mandrills born into a semifree-ranging colony in order to describe growth from birth to adulthood, and to investigate maternal influences upon growth. Adult male mandrills are 3.4 times the body mass, and 1.3 times the CRL, of adult females. Body mass dimorphism arises from a combination of sex differences in length of the growth period (females attain adult body mass at 7 years, males at 10 years) and growth rate. Both sexes undergo a subadult growth spurt in body mass, and this is much more dramatic in males (peak velocity 551 g/months ± 89 SEM at 84–96 months). CRL dimorphism arises from bimaturism (females attain adult CRL at 6 years, males after 10 years), and neither sex shows a particular subadult growth spurt in CRL. Sexual size dimorphism thus represents important time and metabolic costs to males, who mature physically approximately 3–4 years after females. Considerable interindividual variation occurs in the size-for-age of both sexes, which is related to maternal variables. Older mothers have heavier offspring than do younger mothers, and higher-ranking mothers have heavier offspring than do lower ranking mothers. Mass advantages conferred upon offspring during lactation by older and higher-ranking mothers tend to persist postweaning in both sexes. Thus maternal factors affect reproductive success in both sexes, influencing the age at which offspring mature and begin their reproductive career. Am J Phys Anthropol 115:349–360, 2001. © 2001 Wiley-Liss, Inc.