Arteriosclerosis in dogs is not uncommon, although it differs from that seen in man in several respects.The clinically most interesting form is probably the arteriosclerosis seen in the intramural myocardial vessels and brain. We have studied primarily the myocardial small arteries, were we see arteriosclerosis of two types; hyaline material deposited in the vessel wall and a proliferative type were fibroblasts and smooth muscle cells proliferate . This leads to a narrowing of the vessel lumen that could provoke ischemia and possibly risk for microtrombi to form. The aim of our studies has been to describe these changes in naturally occuring heart disese in dogs. The first study was retrospective, and there we could conclude an association of these changes to sudden death and congestive heart failure (CHF). The second study was a case control study were arteriosclerosis in the heart, lung and kidney was studied in dogs with myxomatous mitral valve disease (MMVD) and CHF compared to age and weight matched controls. MMVD-dogs were found to have significantly more arterial narrowing in the left ventricle (p<0.001) lung (p<0.001) and kidney than controls and also significantly more fibrosis in the left ventricle (p<0.001). The changes were also more pronounced in the papillary muscles. The third study was a prospective study were pathology was associated to clinical findings in dogs with naturally occuring CHF. In this study arterial narrowing was associated to fibrosis (p <0.001), echocardiograpkic indices of systolic function ( fractional shortening (p=0.002) ) and mitral regurgitation (proximal isovolumetrc surface area (p=0.03)). This lends support to the hypothesis that naturally occuring CHF in dogs is affected by factors such as arteriosclerosis and fibrosis in the myocardium. References: on request