BACKGROUND: The development of obesity through childhood, often characterized by using body mass index (BMI), has received much recent interest because of the rapidly increasing levels of obesity worldwide. However, the extent to which the BMI trajectory in the first year of life (the BMI 'peak' in particular) is associated with BMI in later childhood has received little attention. SUBJECTS: The Uppsala Family Study includes 602 families, comprising mother, father and two consecutive singleton offspring, both of whom were delivered at the Uppsala Academic Hospital, Sweden, between 1987 and 1995. The children's postnatal growth data, including serial measurements of height and weight (from which BMI was calculated), were obtained from health records. All children had a physical examination when they were aged between 5 and 13 years, at which height and weight were again recorded and used to calculate age- and sex-adjusted BMI z-scores. METHODS: Subject-specific growth curves were fitted to the infant BMI data using penalized splines with random coefficients, and from these the location of the BMI peak for each participant was estimated. A multilevel modelling approach was used to assess the relationships between the BMI peak and BMI z-score in later childhood. RESULTS: The BMI peak occurred, on average, slightly later in female children, with a higher BMI peak in male children. Considered separately, both age and BMI at BMI peak were positively associated with later BMI z-score. Considered jointly, both dimensions of BMI peak retained their positive associations. CONCLUSIONS: The growth trajectory associated with higher childhood BMI appears to include a later and/or higher BMI peak in infancy.