In the present work the examination and comparison of the degree of atherosclerotic involvement of the left coronary artery (CDA) and internal mammary artery (AMI) was performed on 22 unselected subjects at necroscopy examination. The maximum age was 77 and the minimal was 25 (average 62.59 +/- 2.47); 13 were male and 9 female. We macroscopically and microscopically examined three sections obtained from three different segments for each vessel. First we macroscopically evaluated the degree of narrowing of the lumen giving a score (+ to +). (Tab. II). Then the sections, 6 micron thick, stained with Haematoxylin-eosine, Alcian blue, PAS and Van Gieson methods were accurately examined at microscope with a graduated eyepiece. The lumen, the intima and the media were measured at the level of maximum thickness of the intima. Then the thickness of intima and the intima to media ratio, accordingly to the literature, were assumed as major indicators of atherosclerosis involvement. The intima to media ratio average was 5.05 (+/- 5.81) in the CDA vs. 0.44 (+/- 0.29) in the AMI (p less than 0.001). The intima in the CDA was 0.86 mm (+/- 0.76) thick (average) vs. 0.09 mm (+/- 0.07) in the AMI (p less than 0.001). These results show a great difference in atherosclerosis involvement which results to be ten times greater in CDA than in AMI in non selected people. Almost the same result is obtained by the measurement of the intima. On the contrary no detectable difference was found between the media of the two vessels.