OBJECTIVE--To determine the effectiveness of health checks, performed by nurses in primary care, in reducing risk factors for cardiovascular disease and cancer. DESIGN--Randomised controlled trial. SETTING--Five urban general practices in Bedfordshire. SUBJECTS--2205 men and women who were randomly allocated a first health check in 1989-90 and a re-examination in 1992-3 (the intervention group); 1916 men and women who were randomly allocated an initial health check in 1992-3 (the control group). All subjects were aged 35-64 at recruitment in 1989. MAIN OUTCOME MEASURES--Serum total cholesterol concentration, blood pressure, body mass index, and smoking prevalence (with biochemical validation of cessation); self reported dietary, exercise, and alcohol habits. RESULTS--Mean serum total cholesterol was 3.1% lower in the intervention group than controls (difference 0.19 mmol/l (95% confidence interval 0.12 to 0.26)); in women it was 4.5% lower (P < 0.0001) and in men 1.6% (P < 0.05), a significant difference between the sexes (P < 0.01). Self reported saturated fat intake was also significantly lower in the intervention group. Systolic and diastolic blood pressures and body mass index were respectively 1.9%, 1.9%, and 1.4% lower in the intervention group (P < 0.005 in all cases). There was a 3.9% (2.4 to 5.3) difference in the percentage of subjects with a cholesterol concentration > or = 8 mmol/l, but no significant differences in the number with diastolic blood pressure > or = 100 mm Hg or body mass index > or = 30 kg/m2. There was no significant difference between the two groups in prevalence of smoking or excessive alcohol use. Annual rechecks were no more effective than a single recheck at three years, but health checks led to a significant increase in visits to the nurse according to patients' degree of cardiovascular risk. CONCLUSIONS--The benefits of health checks were sustained over three years. The main effects were to promote dietary change and reduce cholesterol concentrations; small differences in blood pressure may have been attributable to accommodation to measurement. The benefits of systematic health promotion in primary care are real, but must be weighed against the costs in relation to other priorities.