Background The rising prevalence of obesity and its putative impact on coronary risk factors threatens the sustained decline in the incidence of coronary heart disease. An understanding of the temporal relationship between obesity and other risk factors is required.Design Independent cross-sectional surveys.Methods We analysed standardized data from five population surveys of risk factors involving Perth men (n=2767) and women (n=2833) aged 35-64 years from 1980 to 1999. Trends in plasma cholesterol, BMI and waist-hip ratio were assessed.Results Striking annual increases were noted in BMI (and waist-hip ratio) of 0.075kg/m(2) in men and 0.083kg/m(2) in women (each P < 0.0001), approximating an 0.8% annual rise in the prevalence of overweight. In contrast, annual mean total cholesterol fell by 0.024 mmol/l in men and 0.030 mmol/l in women (both P < 0.0001), adjusted for confounders. Similarly, the prevalence of cholesterol >= 6.5 mmol/l declined an average of 0.6% per annum in men [odds ratio (OR)= 0.970, 95% confidence interval (CI) 0.954-0.985, P=0.0002)] and by 0.9% in women (OR=0.955, 95% CI 0.938-0.971, P < 0.0001). Decreases in cholesterol were associated with a downward population shift and increasing awareness and treatment of high cholesterol among the elderly.Conclusions Despite rapid increases in adult obesity, plasma mean cholesterol levels have fallen, due possibly to both dietary changes and greater awareness and treatment of high cholesterol in some individuals. This finding fails to confirm a positive association between trends in obesity and plasma cholesterol in the general population.