To clarify the etiology of atherosclerosis in collagen disease, the prevalence and quality of coronary arterial lesions was examined histopathologically. The materials consisted of 68 autopsy cases, including 10 of rheumatoid arthritis (RA), 28 of systemic lupus erythematosus (SLE), 8 of progressive systemic sclerosis (PSS), 5 of dermatomyositis (DM) and 17 of miscellaneous collagen disease (MD). As a control group (C), 9 age-matched cases of hematologic disorders were chosen. In order to conduct systematic research on coronary arteries, tissue blocks were taken, according to the method proposed by the "Vascular Lesion of Collagen Disease Research Committee" in Japan. To estimate the narrowing of the coronary arterial lumen quantitatively, the coronary stenosis index (CSI), which was the sum of the grade of three main coronary arterial narrowing scores, were used. Significant coronary stenosis (more than 75% occlusion of the lumen) was observed in 8 cases of SLE, one of PSS, 2 of DM and 4 of MD. Stenosis was due to atherosclerosis except in 3 cases of MD. The degree of stenosis expressed by the CSI was higher in MD, SLE and DM than in C (p<0.05). Atherosclerotic lesions in collagen disease tended to have a higher population of cellular components than did those in C. There were no statistical correlations between the CSI and some risk factors (age, hypertension, hypercholesterolemia and long-term corticosteroid administration). In the 12 cases with significant stenosis due to atherosclerosis, only 4 patients received corticosteroid hormone for more than one year. Active vasculitis with prominent inflammatory cell infiltration was observed in 2 cases of RA, 3 of SLE and 9 of MD. In cases of vasculitis in SLE examined by the serial section method, luminal narrowing caused by intimal fibrocellular proliferation seemed to have a close relationship with inflammatory cell infiltration in the media and the adventitia. It was concluded that prolonged stimulation of the injured intima by the common risk factors played an important role in the acceleration of coronary atherosclerosis and this intimal change should be reconsidered as a preceding lesion of coronary atherosclerosis.