The British (Tanner and Whitehouse) and American (National Center for Health Statistics, NCHS) growth standards are widely used internationally, although the data are now over 30 years old. Routine weight data was retrieved from the child health records of a complete annual cohort of 3418 children aged 18-30 months to test the validity of these standards for modern infants. Compared with the Tanner and Whitehouse standards, Newcastle children rose initially and then fell a mean of 0.7 SDs between 6 weeks and 18 months, resulting in a threefold difference in the proportion of children below the 3rd centile at different ages. NCHS standards showed a similar pattern. When compared with modern standards from the Cambridge growth study, there was a much closer match, although Newcastle children showed a slight gain by the age of 1 year. Existing standards for weight introduce inaccuracy into the estimation of centile position in the early months of life. As both standards show similar problems this probably represents a real secular change due to changes in infant nutrition. These findings support the need to develop new national growth reference standards.