Endocardial fibroelastosis (EFE) is a pathological disorder characterized by a diffuse and elastic thickening of the endocardium. It is often thought to be congenital as most victims are infants who die of congestive heart failure before reaching one year of age. A naturally occurring animal model of EFE was proposed more than a decade ago on the basis of one pair of adult pure-bred Burmese cats which produced offspring consistently affected with EFE. In the present study a case of EFE is described which occurred in a closed colony of experimental SPF cats. A 5-week-old kitten which was found dead showed hydrothorax, hydropericardium and oedematous lungs at necropsy. Histological examination of the heart revealed left atrio-ventricular dilation with severe, diffuse endocardial thickening which did not extend into the myocardium. A retrospective study was carried out on stored tissues of two further kittens and one adult male cat from the colony which had died during a period of 2 years. The hearts of all the animals showed endocardial thickening in the left atria and ventricle but not in the right chambers. Over a period of 12 years of existence of this colony considerable inbreeding had occurred and it is concluded that an inherited factor was responsible for the disorder.