Osteoporosis is a progressive condition involving structural deterioration of bone tissue, leading to skeletal fragility and an increased susceptibility to fractures due to low bone mass and high rates of bone turnover. Areal bone mineral density (aBMD) serves as the most reliable predictor of susceptibility to osteoporotic fracture. The development of animal models, including Old World Monkeys, has been essential to studies of bone mineral density. These animals, including the baboon, exhibit many biological similarities with our own species relevant to the variation in age-related changes and pathology in bone that may make them an excellent model for studies of skeletal structure and maintenance in humans. The baboon has been shown to exhibit extensive biological similarities to humans regarding skeletal biology, but little is known about the range of normal variation in skeletal traits, such as bone mineral density, in this species. Our data, collected on baboons ( Papio hamadryas) that are part of a large breeding colony at the Southwest Foundation for Biomedical Research and the Southwest National Primate Research Center (San Antonio, TX), involve 466 females and 210 males, ranging in age from 5.5 to 30 years. Student's t tests, bivariate correlations, and likelihood ratio tests show sex and age effects at all spinal sites. Age effects are minimal or absent in the forearm sites. This study is the first to characterize normal variation in aBMD in baboons, to assess the effect of age and sex on this variation, and to compare this variation to those data currently available from experimental control animals. As such, it provides much-needed reference standards that will allow researchers to evaluate the status of their animals in cross-sectional studies and more fully assess the meaning of aBMD changes in longitudinal studies.