Fifty-nine nonobese normal and hyperlipidemic men were instructed in an isocaloric American Heart Association (AHA) fat-controlled diet and followed up monthly for six months. In accordance with the instructions in the AHA diet pamphlets, polyunsaturated fat in excess of minimum requirements was allowed, and carbohydrate intake was not restricted. All serum lipid and lipoprotein concentrations decreased substantially during the first month and were significantly decreased at the end of six months. These decreases (mean ± standard error) were cholesterol 9.6 per cent ± 0.9, β-lipoprotein 15.6 per cent ± 2.1, triglyceride 11.7 per cent ± 3.2, pre β-lipoprotein 5.7 per cent ± 4.8 and α-lipoprotein 6.2 per cent ± 2.5. Body weight was reduced by only 0.8 per cent. Serum urate levels tended to rise, and carbohydrate tolerance was improved. Mean responses were similar in thirty-one “normal” subjects, eight with “type II” hyperlipoproteinemia, eleven with “type IV” hyperlipoproteinemia and nine with “mixed type” hyperlipoproteinemia. Rises in serum triglyceride levels of more than 10 per cent occurred in only thirteen subjects (22 per cent), of whom four had “type IV” hyperlipoproteinemia. On the average, the greatest fall in the levels of serum triglyceride and pre β-lipoprotein occurred in subjects whose carbohydrate intake was substantially reduced. However, declines in the mean levels of these lipids also occurred even among patients whose carbohydrate intake increased. Increases in the linoleate content of plasma phospholipid and of adipose tissue reflected increases in the intake of polyunsaturated fatty acids.