This study was designed to determine effects of reducing intake of total fat and increasing carbohydrate (glucose) on plasma lipoproteins. Eleven men were investigated. They were given two diets for 1 month each. One diet contained 40% of calories as fat with 20% saturated fatty acids, 10% monounsaturates and 10% polyunsaturates. The other diet contained 30% fat with equal amounts of each type of fatty acid. The 10% of fat removed from the latter was replaced by glucose. Six patients had significant reductions of cholesterol in total plasma and low density lipoprotein (LDL) on the 30% fat; for the group as a whole; however, the decrease was not statistically significant. Total triglycerides increased modestly (15%) and high density lipoprotein (HDL)-cholesterol fell significantly (14%) on replacement of 40% fat with 30% fat. Seven patients also were given a 30% fat diet containing fatty acids in the same proportions as in the 40% fat diet. A similar response was noted as when fatty acids were given in equal ratios. This study indicates that response to reduction in fat content is inconsistent. The majority of patients were responders; others, however, were not.