In 82 subjects, aged 21-73, we studied the effect of skim milk supplementation on serum cholesterol concentration, blood pressure, and serum triglyceride level. The study involved a 1-week pretreatment baseline period followed by 8 weeks of milk supplementation. Sixty-four people were designated to a test group and 18 people were placed in a seasonal index group. The study was designed as a free-living trial, i.e., participants were requested to maintain their normal lifestyles, including dietary pattern, except for the supplementation of one quart of 2% solids-not-fat fortified skim milk to the daily diet in the test group. Supplemental milk treatment was associated with a 6.6% reduction (p = 0.0004) of serum cholesterol in the high cholesterol (greater than or equal to 190 mg/dl) test subgroup within the first 4 weeks. No change was noted in serum cholesterol in the low-cholesterol (less than 190 mg/dl) subgroup throughout the study. Body weight and seasonal variation of blood cholesterol did not significantly influence serum cholesterol levels. Reduction (p = 0.0140) in percentage of calories from fat in the high-cholesterol subgroup was not correlated with the decrease in serum cholesterol in this test subgroup. Reductions in systolic and diastolic blood pressure occurred in the test subgroups; the low-cholesterol subgroup had a greater reduction (p = 0.0002) in diastolic blood pressure than the high-cholesterol group (p = 0.0049). Milk supplementation was associated with reduction (p = 0.0370) in serum triglycerides in the high-cholesterol subgroup.