To evaluate the natural history and timing of adiposity rebound (nadir of body mass index (BMI)) in children with congenital adrenal hyperplasia 21-hydroxylase deficiency (CYP21).A retrospective mixed longitudinal study.Height and changes in body composition (BMI; weight (kg)/height2 (m)), triceps and subscapular skinfolds) were analysed in 22 (14 girls, eight boys) prepubertal patients with CYP21 for whom continuous anthropometric data were available for at least seven years. BMI and height SD scores were compared at 1, 5, and 10 years of age. Skinfold SD scores were compared at 2.5 and 5.5 years. Thirteen children (nine girls, four boys) had records available from birth which allowed the estimation of the age at adiposity "peak" and "rebound".A significant increase in BMI SD score was found at 5 and 10 years compared with those at 1 year. No significant change in height SD score was observed at these ages. Triceps and skinfold SD score were increased significantly at 5.5 compared with 2.5 years. The "rebound" in BMI SD score took place at 1.74 years (range 0.71-4.57) compared with 5.5 years (range 3.5-7.0) in the normal UK populations.Normally growing patients with CYP21 increased their BMI throughout childhood. Adiposity rebound took place on average three years earlier than in the general population. These findings suggest that even when well controlled in terms of their disease process, patients with CYP21 are at risk of obesity, which may have important implications for the evolution of reproductive function (polycystic ovaries), diabetes, hypertension, and cardiovascular disease in these subjects.