It has been reported that lipid-lowering diets may reduce high density lipoprotein as well as low density lipoprotein cholesterol but that the reduction of the high density lipoproteins is less pronounced with diets of lower P/S ratios. The aim of the present study was therefore to investigate whether the effects on the low density lipoproteins could be maintained, but those on the high density lipoproteins diminished, by reducing the P/S ratio from 1.3 to 0.7. Fourteen hyperlipoproteinaemic patients in a metabolic ward were given two fat-modified diets during two consecutive 3-week periods in a randomized order. The diets were identical with regard to nutrient composition but differed with regard to the P/S ratios, which were 1.3 and 0.7, respectively. The average serum lipoprotein lipid composition and the apolipoprotein concentrations were similar at the end of the two periods. However, in the group of patients (n = 7) who started on the diet with P/S ratio 1.3 the low density lipoprotein cholesterol increased significantly (p less than 0.05) by 7% and the apolipoprotein B concentration by 10% (p less than 0.001) when shifting to the diet with P/S ratio 0.7. No changes of the high density lipoprotein concentrations were seen. The relative content of saturated fatty acids, of oleic acid, and of arachidonic acid in the plasma lipid esters decreased, but the content of linoleic acid increased, when shifting from the diet with a P/S ratio of 0.7 to that of 1.3.(ABSTRACT TRUNCATED AT 250 WORDS)