BACKGROUND: Dietary factors are an important contribution to the high rates of coronary heart disease in the UK. One approach to achieving change is health-promoting advice in primary care. AIM: To compare the effectiveness of structured dietary advice by practice nurses with standard health education in changing serum cholesterol, weight and diet. METHOD: Randomized, controlled trial within eight general practices in England and Wales allocated within matched geographical pairs to 'dietary advice' or 'usual care'. Men and women aged 35-59 years, recruited opportunistically by their GPs, underwent health checks. In 'dietary advice' practices, subjects received dietary advice from specially trained nurses based on negotiated change principles, reinforced at follow up. In 'usual care' practices, subjects were only given standard health education materials. RESULTS: A total of 956 patients were recruited: 473 in 'dietary advice' practices and 483 in 'usual care' practices. Compliance with annual follow up was 80%. Compared with 'usual care' practices, there was a mean 0.20 mmol/l lower serum cholesterol (95% CI -0.38 to -0.03 at 1 year) in 'dietary advice' practices. There was a small fall in weight of 0.56 kg (95% CI -1.04 to -0.07) and reductions in total and saturated fat. Factor VII coagulant activity fell by a mean of 6.7% of the standard (95% CI -15.4 to +2.0). CONCLUSION: Provision of standard health education material alone as part of a health check had no effect on coronary heart disease risk factors. There were modest changes in diet and associated risk factors when a more intensive and individual approach to dietary advice was given by practice nurses. This is, however, probably an ineffective use of resources, except in those at high risk of coronary heart disease. Whole-population strategies to achieve dietary change are required.