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Background - Although several studies have shown that an early adiposity rebound (AR) is associated with an increased risk of adult obesity, it is not clear whether the rebound in body mass index (BMI) is actually attributable to changes in body fat. Objective - To determine the changes in body composition occurring in girls during AR. Design - Body composition was measured using dual-energy x-ray absorptiometry (DXA) at baseline, year one, year two and year four in 40 girls aged 3-6 years at baseline. Dietary intake was estimated by four-day diet records and physical activity by questionnaire. Age at AR was determined by modelling. The velocity of change (% per year) in height, weight and fat and lean tissues was estimated for each subject using random coefficient models. Early AR was defined as less than five years of age and late AR as five or more years of age. Outcomes - Early and late rebounders did not differ in age, height, weight, body composition, dietary protein intake or physical activity participation at baseline. Although height velocity was similar, weight velocity was significantly higher in early compared with late rebounders (13.5% vs 10.7, P<0.001). Differences in weight velocity were entirely due to change in body fat stores; early rebounders gained body fat at more than twice the rate of late rebounders (17.1 vs 6.5%, P<0.001) whereas no differences were observed in lean mass velocity. By the age of 9 years, girls with an early AR were more than 4 kg heavier than girls with an early AR, with considerably more body fat and a greater percentage classified as overweight. Conclusions - Our study demonstrates that the differences in BMI during AR are due specifically to alterations in body fat and not to alterations in lean mass or height. [ABSTRACT FROM AUTHOR]