Pathohistological and biochemical studies were conducted on the severity of arteriosclerosis in the internal thoracic artery (ITA), an artery commonly used for coronary artery bypass grafting (CABG). For the pathohistological examination, 26 bilateral ITAs and 13 left anterior descending coronary arteries (LADs) obtained in full length from 13 autopsy cases, none of which had died of arteriosclerotic heart disease, were used. The ratio of the thickness of the intima to that of the media (R) was used as the index for arteriosclerosis. ITAs and LADs were classified as grades I to IV according to the value of R. The R of the ITAs was approximately 1/10 that of the LADs (P < 0.01). Most ITAs showed a low arteriosclerotic grade, with no variation in arteriosclerosis along their length and a low R in all segments. No difference was found between right and left ITAs. Biochemical examination was conducted on 12 ITAs and 11 LADs, obtained from 12 different and unselected autopsy cases. The lipid content in the vascular wall was determined to evaluate the severity of arteriosclerosis, with the following results: Total cholesterol, 5.5 +/- 1.8 and 17.8 +/- 13.6 micrograms/mg wet weight (P < 0.05); triglyceride, 90.4 +/- 90.3 and 114.4 +/- 117.2 micrograms/mg wet weight (n.s.); and phospholipid, 7.4 +/- 3.9 and 11.2 +/- 3.9 micrograms/mg wet weight (P < 0.05), respectively, for the ITAs and LADs. These findings thus demonstrate that arteriosclerosis of the ITA in Japanese people is very mild, compared to that of the LAD in the same individuals.