Fat mass was recently shown to be a positive determinant of bone mass and size independently of lean mass in a birth cohort of British 9-year-olds. The present study was undertaken to investigate whether similar relationships are evident in younger, preschool children. Height and weight were measured, and a total-body dual-energy X-ray absorptiometric scan was performed on 194 preschool New Zealand children (81 girls, 113 boys) participating in the Dunedin birth cohort Family, Lifestyle, Activity, Movement, and Eating (FLAME) study close to their fifth birthday. Relationships of total-body fat mass and lean mass to total-body-less-head (TBLH) bone area and TBLH bone mineral content (BMC) were evaluated using linear regression. Girls had higher mean fat mass (3.9 vs. 3.2 kg) and lower lean mass (14.5 vs. 15.2 kg) than boys (P < 0.001), but their heights, weights, and TBLH bone area were similar. Although a given weight of lean tissue was associated with greater increases in TBLH area than a given weight of fat tissue, our results show that fat mass was an independent predictor of TBLH bone area (R (2 )= 0.79, P < 0.001) and TBLH BMC (R (2) = 0.74, P < 0.001) in data adjusted for socioeconomic status, ethnic group, lean mass, and height. We conclude that increased fat mass is associated with outward expansion of the TBLH skeletal envelope (wider bones) independently of height and lean mass in very young children.