This study examined heart tissue samples from 16 dogs and 13 cats to investigate whether chlamydial infections are correlated with coronary arteriosclerotic lesions in canines and felines. Real-time PCR, microarray, and immunohistochemistry testing found chlamydia in some samples but no obvious connection between spontaneous lesions and infections. While arteriosclerosis was present in all cat hearts, only two dogs showed atherosclerosis, and chlamydia was not correlated with these solely atherosclerotic cases. More sample testing is needed to make definitive conclusions about links between chlamydial infections and canine or feline coronary arteriosclerosis.