Weight gain between birth and 9 months of 12 903 term Millennium Cohort Study infants was investigated in order to determine differences according to sex, ethnicity and country of birth. The standardised weights and weight gains were also compared with a cohort of mainly white infants born 10 years earlier to determine whether weight gain has changed over the last decade. There were significant differences between ethnic groups, with black infants showing the largest weight gain and Asians the smallest. White boys born in England and Scotland grew relatively faster than girls, but there were no significant gender differences among the other ethnic groups or among infants born in Ireland and Wales. There was very little difference in weight gain between white English Millennium cohort infants and the earlier cohort, suggesting that the current epidemic of childhood obesity starts after 9 months of age.