In some good responders to dietary treatment of hyperlipidemia followed for four years (the Oslo Study of men aged 40 to 49 years), a significantly increased high density lipoprotein (HDL) cholesterol concentration was found in the treated men as compared to an untreated control group, well matched for pretreatment values of HDL cholesterol, triglycerides and other relevant variables. In the treated group, diet analysis at four years revealed an isocaloric diet, with 27.9 per cent of total calories from fats, 8.2 per cent of total calories from saturated fatty acids and a polyunsaturated to saturated ratio (P:S ratio) of 1.01—as compared to 44.1, 18.3 and 0.39, respectively, in the control group. After four years of observation the mean total cholesterol in the treated group was 263 mg/dl versus 341 mg/dl in the control group, mean fasting triglycerides 129 mg/dl versus 200 mg/dl, whereas mean HDL cholesterol was approximately 20 per cent higher in the treated group than in the control group (50.09 mg/dl versus 42.22). The “HDL ratio” (HDL cholesterol × 100 to total cholesterol — HDL cholesterol) was 69 per cent higher in the treated group. HDL cholesterol was significantly and positively correlated with the P:S ratio in the diet. In this investigation only good diet responders have been studied. On the other hand, a clearly significant increased HDL cholesterol has been demonstrated in the dieters as compared to well matched control subjects. These results might be of interest in the view of HDL cholesterol as a well documented and independent “antirisk factor” for coronary heart disease.