In order to assess the extent to which children in the UK will follow the UK-WHO head circumference standard, the authors used head circumference data from the Southampton Women's Survey (n=3159) and the Avon Longitudinal Study of Parents and Children (n=15 208) in children aged 0-36 months, converted into z-scores using both the UK-WHO and UK1990 references. Rapid head growth was defined as crossing upwards through two major centile bands (1.33 SD). The UK-WHO standard identified many more infants with heads above the 98th centile than to the UK1990 reference (UK-WHO: 6-16% of infants at various ages; UK1990: 1-4%). Rapid head growth in the first 6-9 months was also much more common using the UK-WHO standard (UK-WHO: 14.6-15.3%; UK1990: 4.8-5.1%). Practitioners should be aware of these findings to avoid unnecessary referrals.