Body composition was assessed in 68 children and adolescents with diabetes mellitus. Body fat was measured by skinfold thickness and the results were compared with those obtained by bioelectrical impedance methods (r = 0.76, p less than 0.001). Percentage body fat from skinfold thickness estimations was 12-38%. Girls appeared to be most at risk of obesity during puberty with mean body fat 31%, significantly greater than in prepubertal girls (22%, p = 0.001) and pubertal boys (21%, p less than 0.001). Body fat increased progressively through puberty in girls, with those in late puberty (stages 4 and 5) having significantly more percentage body fat than those in earlier puberty (stages 2 and 3, p less than 0.001). Those in late puberty also received significantly more insulin per kilogram body weight (p = 0.05) and had higher HbA1 levels (p less than 0.05) than prepubertal girls.