The relationship between diet, serum lipoproteins, and the progression of coronary lesions was studied in 39 patients with stable angina pectoris in whom coronary arteriography had shown at least 1 vessel with 50% obstruction before intervention. Intervention consisted of a 2-year vegetarian diet that had a ratio of polyunsaturated to saturated fatty acids of at least 2.0, and that contained less than 100mg of cholesterol per day. Dietary changes were associated with a significant increase in linoleic acid content of cholesterol esters and a significant lowering of bodyweight, systolic blood pressure, serum total cholesterol, and the ratio of total to high density lipoprotein (total/HDL) cholesterol. Angiographic examination was performed after 24 months; angiograms were assessed visually (with blinding) and by computer-assisted image analysis. Both types of assessment indicated progression of disease in 21 of 39 patients but no lesion growth in 18. Coronary lesion growth correlated with total/HDL cholesterol (r = 0.50, p = 0.001) but not with blood pressure, smoking status, alcohol intake, weight, or drug treatment. Disease progression was significant in patients who had values for total/HDL cholesterol that were higher than the median (greater than 6.9) throughout the trial period. No coronary lesion growth was observed in patients who had lower values for total/HDL cholesterol (less than 6.9) throughout the trial or who initially had higher values (greater than 6.9) that were significantly lowered by dietary intervention.