We investigated 11 beta hydroxysteriod dehydrogenase type 1 (11betaHSD-1) sequence variants in 103 healthy overweight (BMI >2 s.d.) and 160 nonoverweight (BMI -2 to +2 SD) children to examine the associations between body composition and 11betaHSD-1 polymorphisms. A total of 4.3% of children were homozygous and 30.0% heterozygous for an adenine insertion in intron 3 (ins4436A). By ANCOVA (adjusting for age, sex, race, and height), BMI-s.d. differed according to ins4436A genotype (P<0.005), with the greatest BMI-SD for ins4436A homozygotes (mean +/-s.d., 3.4+/-3.4, vs heterozygotes, 0.8+/-5.5, or wild-type, 1.8+/-7.5). Homozygotes also had greater waist circumference, waist-to-hip ratio, and insulin resistance indices than heterozygote or wild-type children (all P<0.05), but no significant differences in trunk fat by DXA, or in serum lipids. We conclude an intronic 11betaHSD-1 gene polymorphism is associated with greater body mass, altered body composition, and insulin resistance in children. 11betaHSD-1 may be one of the genes relevant for pediatric-onset obesity and its complications.