Objective To describe infant growth in relation to breastfeeding intensity and to investigate the relation between breastfeeding intensity and infant health in months 1 to 9.

Methods Mothers who delivered a baby in April 2005 in Bavaria, Germany, were recruited for a prospective cohort study in 141 clinics, 10 birth houses and by home-birth assisting midwives. Health and weight data assessed by a physician and detailed breastfeeding data were available from questionnaires at the age of 2–6 days, 2, 4, 6 and 9 months. Subjects were healthy term infants weighing ⩾2500 g at birth. We compared 475 infants breastfed exclusively for ⩾6 months (group A), 870 infants breastfed fully or exclusively ⩾4 months with continued exclusive/full/partial or no breastfeeding until month 6 (group B) and 619 infants breastfed <4 months or not breastfed (group C).

Results Multivariate analysis showed that group A had a significantly reduced risk for ⩾1 episode of GIT-infection during months 1–9 compared to group C (adjusted OR, 0.60; 95% CI 0.44 to 0.82; p<0.01), whereas a protective effect for group B was not significant in multivariate analysis. Applying WHO-child-growth-standards we found lower weight-for-length z-scores in the first days of life in group C vs. A and B. In month 6 or 7 group C had the highest weight-for-length z-scores.

Conclusion Our findings support the recommendation for ⩾6 months of exclusive breastfeeding; causality cannot be derived from our findings. Research is needed to explain the differences in child growth between the groups.