Although exclusive breastfeeding for the first 6 months of infant life is recommended in the UK, there is little information on the extent of exclusive breastfeeding. This study has taken the 1996 and 2003 World Health Organization (WHO) definitions of breastfeeding and investigated breastfeeding rates in the first 6 months of life in infants born to mothers enrolled in a longitudinal, representative, population-based cohort study – the Avon Longitudinal Study of Parents and Children (ALSPAC). Information about breastfeeding and introduction of solids was available for 11 490 infants at 6 months of age (81% of live births). Exclusive breastfeeding declined steadily from 54.8% in the first month to 31% in the third, and fell to 9.6% in the fourth month mainly due to the introduction of solids to the infants. In the first 2 months, complementary feeding (breastmilk and solid/semi-solid foods with any liquid including non-human milk) was used in combination, and declined from 22% in the first month to 16.8% in the second due to a switch to exclusive commercial infant formula feeding. Replacement feeding (exclusive commercial infant formula or combined with any liquid or solid/semi-solid food but excluding breastmilk) increased steadily from 21.9% in the first month to 67.1% by the seventh. This obscured the change from exclusive commercial infant formula feeding only to commercial infant formula feeding plus solids/semi-solids, a change which started in the third month and was complete by the fifth. Using categories in the 1996 and 2003 WHO definitions, such as complementary feeding and replacement feeding, presented difficulties for an analysis of the extent of breastfeeding in this population.