To monitor growth trends in young children in order to ascertain success (or otherwise) in halting the rapid rise in childhood obesity prevalence, and to assess the suitability of using routinely measured data for this purpose.

Retrospective serial cross-sectional analyses of the proportion of obese children (logistic regression) and BMI standard deviation score (linear regression/maps) were undertaken. BMI coverage was calculated as percentage of sample with data (‘usual’), percentage of total births and percentage of census values. BMI was standardised for age and sex (British reference data set).

Metropolitan Leeds, UK.

Children aged 3 to 6 years. Weight, height, sex, age and postcode data were collected from Primary Care Trust records.

Data were collected on 42 396 children, of whom 13 020 (31 %) were excluded due to missing data/data problems. Seventy-two per cent of 3-year-olds and 92 % of 5-year-olds had data recorded (‘usual’ coverage). From 1998 to 2003 there was a significant increase in the proportion of obese children (4·5 % to 6·6 %; P < 0·001); children were 1·5 times more likely to be obese in 2003 than in 1998.

Childhood obesity rose significantly between 1998 and 2003. Routinely measured data are an important means of monitoring population-level obesity trends, although more effort is required to reduce the quantity of data-entry errors, for relatively low marginal cost.