Total cholesterol, HDL cholesterol, and triglycerides were measured in 31 premenopausal women randomized into one of two diet groups: one diet with a P:S ratio of 1.0 and one diet with a P:S ratio of 0.3. Both groups were fed a high-fat diet (40% of energy from fat) for four menstrual cycles per subject followed by a similar interval on a low-fat diet (20% of energy from fat). Changing from the high-fat to the low-fat diet resulted in a nonsignificant mean decrease of 7% in total cholesterol. HDL-cholesterol response to the low-fat regimen was influenced by the P:S ratio. Women in the high P:S group showed no change; mean HDL cholesterol in women in the low P:S group decreased 12%. Plasma triglycerides increased in both groups on the low-fat diet although the increase was greatest in the low P:S group.