Diffuse intimal thickening (DIT) is a thickened intima present in human arteries before atherosclerosis develops and is considered to be related to atherogenesis. The purpose of this study was to clarify the systemic and age distributions of DIT. Coronary, cerebral, carotid, subclavian, iliac and abdominal organ arteries and the aorta were examined in 72 autopsy cases (aged 36 weeks of gestation to 30 years at death). DIT was found in the coronary arteries and aorta from 36 weeks of gestation and the first year of life, respectively. The intima/media (I/M) ratio of coronary arteries showed an age-dependent increase and was much greater than that of other muscular arteries, i.e., intracranial and extraparenchymal cerebral arteries and abdominal organ arteries. Aorta also demonstrated age-dependent as well as site-dependent increases of I/M ratio; the more distal the segments, the greater the ratio. Consequently, the abdominal aorta had the largest I/M ratio within the aorta. Other elastic arteries, i.e., carotid, subclavian and iliac arteries, showed trends similar to the distal portions of the aorta. Thus, DIT was strongly expressed from an early age in arteries that are considered to be prone to atherosclerosis. These findings suggest that the development of atherosclerosis depends at least partly on the degree of DIT.