The effects of orally administered dilazep, an antianginal drug, on the hemoglobin-oxygen affinity were studied in 31 cases with ischemic heart disease. Prior to medication, the mean P₅₀ value was 29.2±1.65mmHg. There were no significant differences in the P₅₀ value according to the age of patient or the severity of the coronary arterial disease. Acute effects of dilazep were studied in 29 patients. The 30 min and 60 min post-administration P₅₀ values increased significantly to 30.2±2.55mmHg and 30.4±2.31mmHg, respectively. Eight patients were administered 300mg of oral dilazep daily for 4 weeks. Three of 4 in whom exercise tolerance improved showed increases in P₅₀. No changes in various factors which might affect the P₅₀ value, including 2, 3-DPG, were found.