Hyperhomocysteinemia is currently regarded as a risk factor for the development of ischemic heart disease (IHD). We examined the relation between plasma total homocysteine (tHCY) concentrations and the severity and morphology of coronary stenosis in 238 Japanese patients. Coronary stenosis score (CS) in the first quartile of plasma tHCY levels was 4.9 and the second, third and fourth quartiles were 6.5, 7.5, and 8.9, respectively and plasma tHCY levels were 8.5, 10.9, 13.5, and 22.4 mumol/l, respectively. CS was higher in the fourth quartile than in the first, suggesting that plasma tHCY levels are related to the coronary stenosis severity. Among 238 patients, 123 did not have any significant coronary stenosis (group 1), 98 had focal coronary stenosis (group 2) and 17 had diffuse coronary stenosis (group 3). Plasma tHCY level was higher in group 3 than in group 1 or 2, suggesting that plasma tHCY levels are related to the diffuse coronary stenosis. We also examined whether hyperhomocysteinemia is concerned in coronary spasm. Plasma tHCY levels did not differ between 43 patients with vasospastic angina and 43 patients without ischemic heart disease, suggesting that plasma tHCY levels are not concerned in coronary spasm. Therefore, in Japanese patients, plasma tHCY contributes to the development of IHD through the influence on chronic coronary atherosclerotic background, but not through the influence on coronary spasm, i.e., one of the acute ischemic stimuli. Whether or not tHCY influences coronary thrombosis should be clarified in Japanese patients.