OBJECTIVE: Patterns of obesity are an important public health consideration. The purpose of this study was to determine the past and future population trends of obesity in the United Kingdom (UK). METHODS: Data describing body mass index (BMI) were abstracted from the English Health Survey's from 1991 to 2002, inclusive. The UK was assumed to follow the same pattern of change in body composition as England. The distributional pattern of BMI was determined and age and sex specific changes in the distribution described over the period. Data were standardized to the UK population in 2001. Ethnicity was accounted for in the forecast using general linear models for age and sex specific groups applied to government forecasts of future demography. RESULTS: BMI increased by a mean value of 1.21kg/m2 (standard error of the mean (SEM) 0.105 to 0.044 [=18 years]), equivalent to an annual increase of 0.11 kg/m2 per year (95% CI 0.106 to 0.123). BMI in women increased on average from 25.6 to 26.8 kg/m2, and men's from 25.9 to 27.1 kg/m2. This varied by age and sex. The annual change in mean BMI was linear in every age group. In 1991 and 2002 the proportion of overweight (=25.0 to <30.0kg/m2) and obese (=30.0 kg/m2) people in the population rose from 29.5% and 12.0%, to 31.3% and 18.4%, respectively. The number of overweight and obese people in the general population in 2004 was estimated to be 31.2 million people (52.6%), increasing to 38.0 million people (62.0%) in ten years time, an increase of 6.8 million people. CONCLUSION: If reliable, this estimate clearly represents a serious public health challenge.