Familial canine dermatomyositis is a recently identified disease of collie dogs that resembles human juvenile dermatomyositis. The lesions in the skin and muscles obtained by biopsy from two litters of dogs were characterized for the purpose of determining the similarity of the lesions to those of human dermatomyositis. The cutaneous lesions began between 7 and 11 weeks of age and were present on the face, lips, ears, and skin over bony prominences of the limbs, feet, sternum, and tip of the tail. Histologically the cutaneous lesions frequently consisted of vesicles, pustules, and ulcers on the lips, face, and ears. Neutrophils, lymphocytes, mast cells, and macrophages were present throughout the dermis. Neutrophils and lymphocytes were also present in and around vessels. Between 13 and 19 weeks of age generalized muscle atrophy was noted. The muscle lesions consisted of interstitial lymphocyte, plasma cell, macrophage, and neutrophil accumulation; myofiber degeneration, regeneration, and atrophy; and fibrosis. Perivascular neutrophils, lymphocytes, and plasma cells were also seen. Histologically, the lesions resembled those present in human juvenile dermatomyositis; and these observations, coupled with clinical, immunologic, and clinical pathologic observations presented elsewhere, suggest that familial canine dermatomyositis is an appropriate and potentially useful model for human juvenile dermatomyositis.