Context: Intracellular fat within muscle and visceral tissue has been suggested to adversely influence bone development. Objective: The aim of the study was to evaluate associations between im fat, as reflected by muscle density as measured by peripheral quantitative computed tomography, and cortical bone parameters in young adults. Design/Setting/Participants: We conducted a cross-sectional analysis of 1703 males and 2243 females aged 17.8 years from the Avon Longitudinal Study of Parents and Children. Outcome Measures: We measured cortical bone parameters from midtibial peripheral quantitative computed tomography scans. Results: Muscle density (inversely related to im fat) was inversely associated with periosteal circumference (PC) (beta = -0.07 [95% confidence interval (CI), -0.1, -0.04]), cortical bone mineral density (BMD C) (beta = -0.21 [95% CI, -0.26, -0.17]), and cortical thickness (CT) (beta = -0.37[95% CI, -0.42, -0.33]) (males and females combined, adjusted for age, height, gender, and muscle cross-sectional area). In contrast, sc fat area was positively associated with PC (beta = 0.10 [95% CI, 0.07, 0.12]), but no association was seen with BMDC or CT. To examine the role of candidate intermediary metabolic pathways, analyses were repeated after adjustment for insulin, C-reactive protein, and β-C-telopeptides of type I collagen. Whereas similar associations were observed after adjustment for insulin and C-reactive protein, the association between muscle density and BMDC was partially attenuated by adjustment for β-C-telopeptides of type I collagen (beta = -0.14 [95% CI, -0.20, -0.08]). Conclusion: Although im and sc fat were both positively associated with cortical bone mass, the nature of these relationships differed in that im fat was predominantly associated with CT and BMDC, whereas sc fat was mainly associated with PC. These relationships were largely independent of candidate metabolic pathways, such as altered bone resorption, insulin resistance, or inflammation. Copyright