OBJECTIVES: To evaluate changes in plasma cholesterol following health screening and health discussions in general practice. DESIGN: Randomised prospective population-based study conducted over a period of 5 years. SETTING: Primary care, all general practitioners (GPs) in a well-defined area. SUBJECTS: A random sample of inhabitants aged 30-49 years in January 1991, registered with a local GP was invited to participate. The participants (1507 persons, or 75.4% of the 2000 invited) were randomly allocated to two intervention groups and a control group. MAIN OUTCOME MEASURES: Plasma cholesterol, percentage of subjects with plasma cholesterol higher than 7 mmol/l. RESULTS: After 5 years of intervention, plasma cholesterol in the whole population was significantly lower in the intervention groups compared to the control group. The decrease was most pronounced (0.5 mmol/l) in subjects at high cardiovascular risk. The percentage of high-risk individuals with a cholesterol level higher than 7 mmol/l was significantly lower in the intervention groups compared to the control group (9.8% vs 6.2%, p = 0.04), corresponding to a 37% reduction. CONCLUSIONS: The study shows that the health checks had a measurable impact on plasma cholesterol levels, the most pronounced effect is seen among individuals at high cardiovascular risk.