OBJECTIVE:To study the demographic and clinical parameters of three different categories of obesity, with particular focus on a cohort of individuals with BMI > or = 50 kg/m2, the fastest growing category of obesity. DESIGN:Over 700 obese individuals were studied (186 with BMI = 30-39 kg/m2, 316 with BMI = 40-49 kg/m2 and 290 with BMI > or = 50 kg/m2). RESULTS:Median BMI was 51 kg/m2 for patients who reported onset of overweight before 15 years of age, 47 kg/m2 for patients who reported onset between 15 and 30 years, and 42 kg/m2 for patients who became overweight after 30 years of age. The BMI > or = 50 kg/m2 group was notably younger than the group with BMI = 30-39 kg/m2 (44 (SD 11) years v. 50 (SD 15) years; P < 0.0001). Eighteen per cent of obese patients studied were considered metabolically healthy according to standard cut-off points for blood pressure, fasting glucose and lipid profiles. However, the proportion of metabolically healthy individuals was significantly higher in the BMI = 30-39 kg/m2 group than in the BMI = 40-49 kg/m2 and BMI > or = 50 kg/m2 groups (31% v. 17% and 12% respectively; P < 0.05 and P < 0.005). When compared with people of similar age in the general population, individuals with BMI > or = 50 kg/m2 had lower rates of marriage (51% v. 72%) and a higher prevalence of unemployment (14% v. 5%). CONCLUSIONS:The current study suggests that the increasing prevalence of childhood obesity worldwide will lead to many more individuals achieving a higher BMI at a younger age. Furthermore, an earlier onset of overweight does not appear to prevent the adverse metabolic health outcomes associated with extreme obesity.