To access publisher full text version of this article. Please click on the hyperlink in Additional Links field OBJECTIVE: To describe the association between duration of exclusive breast-feeding (EBF), weight gain in infancy and childhood BMI in two populations with a long duration of EBF. DESIGN: Cohort study with follow-up in childhood. Breast-feeding status was reported monthly during infancy. Weight and length were measured at birth, 2, 6 and 12 months of age, as well as in childhood at 6 or 10 years of age. SETTING: Iceland and Denmark. SUBJECTS: Randomly selected healthy newborns from Denmark (n 85) and Iceland (n 100). RESULTS: Infants exclusively breast-fed for < or =2 months gained 348 (95% CI 69, 626) g more weight from 2 to 6 months than infants exclusively breast-fed for 3-4 months (P = 0.009). Weight gain from 6 to 12 months was found to be greater among infants exclusively breast-fed for < or =2 months compared with those exclusively breast-fed for > or =5 months (P = 0.008). A greater weight change, in terms of Z-score, between the ages of 2 and 6 months was associated with higher Z-score of childhood BMI, adjusted for birth weight, country and duration of EBF (B = 0.49, se = 0.11, P < 0.001, adj. R2 = 0.15). However, the association was much stronger in the Icelandic cohort than the Danish one. CONCLUSIONS: Although duration of EBF was not associated with childhood BMI in the present study it may modulate growth rate in infancy, which is related to childhood BMI. However, other factors determinative for infant growth also need to be considered when assessing the relationship of early growth and nutrition to childhood overweight, as traditions in complementary food might differ between populations.