BACKGROUND/OBJECTIVES: Knowledge of longitudinal body composition development is required to identify the mechanisms behind childhood overweight and obesity and to prevent these conditions. However, accurate data on this development in early childhood are lacking. Our aim was to describe the longitudinal body composition development in healthy young Swedish children. SUBJECTS/METHODS: Body size and composition were assessed in 26 children using air-displacement plethysmography (1 and 12 weeks and 4.4 years of age) and isotope dilution (1.5 and 3 years of age) and compared with available reference data. RESULTS: Body fat (%) for boys (n = 16) was 12.8 +/- 3.9 (1 week), 25.6 +/- 4.8 (12 weeks), 28.2 +/- 3.8 (1.5 years), 27.3 +/- 5.1 (3 years) and 26.1 +/- 3.5 (4.4 years). For girls (n = 10) these values were 15.3 +/- 2.9, 25.7 +/- 3.9, 27.9 +/- 3.3, 26.3 +/- 7.2 and 26.0 +/- 5.3, respectively. These values were above the Fomon reference values at 1.5 years of age and later and higher than the Butte reference (Po0.05) for boys at 1.5 years of age. At all ages the coefficients of variation were higher for body fat (%) (12-30%) than for BMI (4-11%). CONCLUSIONS: At 4 years of age our children had more body fat than indicated by reference data. This high level may have already been established at 1.5 years of age but our small sample and the lack of appropriate reference data limit the possibility of drawing firm conclusions. Our results demonstrate the limitations of BMI when investigating overweight and obesity in early life and highlight the need for appropriate reference body composition data in infants and young children.