To investigate the relationship between breastfeeding and infant health and to describe growth in the first 9 months. Mothers delivering a baby in April 2005 were recruited throughout Bavaria, Germany, for a prospective birth cohort study. These mothers reported breastfeeding data, health and growth data of 1901 infants assessed by a physician in questionnaires on day 2-6, and in months 2, 4, 6 and 9. Subjects were healthy term infants with a birth weight > or =2500 g. We compared 475 infants breastfed exclusively for > or =6 months (group A), 870 infants breastfed fully/exclusively > or =4 months, but not exclusively > or =6 months (group B) and 619 infants not breastfed/breastfed <4 months (group C). In multivariate analysis > or =6 months of exclusive breastfeeding reduced significantly the risk for > or =1 episode of gastrointestinal infection(s) during months 1-9 compared to no/<4 months breastfeeding (adjusted odds ratio [OR]: 0.60; 95% confidence interval [CI]: 0.44-0.82). The application of the World Health Organization (WHO)-child growth standards showed lower weight-for-length z-scores in first days of life in group C versus groups A and B, whereas in months 6/7 group C showed the highest scores. Differences in child growth depending on breastfeeding duration should be investigated further. Concerning health outcomes our findings support the recommendation for > or =6 months of exclusive breastfeeding.