The clinical signs, pathology and breeding data of two cases of cardiomyopathy of an unusual kind in adult Holstein Friesian cattle in Britain are reported and compared with those in similar entities in Switzerland, Japan and Canada. The principal and primary lesions affect the heart and these produce secondary changes, particularly in the liver, and result in fatal congestive heart failure. The cardiac lesions consist of extensive myocyte vacuolation, endomysial and perimysial fibrosis and focal cardiac myocyte degeneration, atrophy and hypertrophy resulting in an extended range of myocyte size. Lesions affected all four heart chambers but were most severe in the ventricles. Vascular lesions, particularly moderate medial hypertrophy and intimal thickening of arterioles and arteries, occurred in heart, lung, kidney and lymph nodes. The liver showed severe fibrosis, chronic congestion and hepatocyte loss. There was a chronic multifocal nephritis. The cause is unknown, but the affected animals were full brother and sister and have a common ancestor in the male and female line five generations earlier.