The differential effect of two diets, taken in synchrony with the menstrual cycles for 2 wk each, on serum and bile lipids was investigated in young healthy women. The "normal" diet was high in cholesterol and total fat, and low in polyunsaturated fat and fiber; the "prudent" diet contained a high proportion of polyunsaturated fat and fiber, but was low in cholesterol and total fat; there was little difference in energy content. Both in whole serum and in low-density lipoprotein the concentrations of cholesterol and apolipoprotein B were almost 30% lower with the "prudent" than with the "normal" diet; HDL-cholesterol was 16.3% lower. Triglycerides were increased, only in the very-low-density lipoproteins while cholesterol and apolipoprotein B did not change much in this fraction. The risk to acquire cholesterol gallstones was not less with the use of the "prudent" diet as originally expected. While using the "prudent" diet five of the women had slightly higher lithogenic indices, in two there were much higher values (greater than 25%), and only in three the lithogenic index was unchanged or slightly lower than with the "normal" diet.