One hundred and ten adult out-patients entered a prospective study of the effect of dietary control of hyperlipidaemia. Seventy-nine were male and 31 female. Eighty-four patients had primary type II and 26 had primary type IV hyperlipidaemia. Cases of secondary hyperlipidaemia were excluded. Patients with type II hyperlipidaemia were treated with a diet low in saturated fat and cholesterol, with substitution of polyunsaturated fats. Additional carbohydrate and alcohol restriction was applied to patients with type IV hyperlipidaemia. Calorie restriction was prescribed for overweight patients. In type II patients, mean serum cholesterol fell from 322 to 286 mg per 100 ml, and weight from 159 to 152 lb over the first six months of treatment. In type IV patients cholesterol fell from 328 to 281 mg per 100 ml, triglycerides from 389 to 241 mg per 100 ml and weight from 166 to 156 lb over the same period. All reductions were statistically significant at the 1 per cent level. No statistical correlation was observed between reduction of weight and reduction in lipid levels. Over a three-year follow-up period no tendancy for lipid levels or weight to 'rebound' was noted. It is concluded that diet can produce a significant reduction in plasma lipid levels over six months in some hyperlipidaemic patients. Further follow-up is continuing to determine whether this effect can be maintained over the longterm.