Spontaneous acute lymphoblastic leukemia (ALL-SD) was diagnosed in a breeding colony of isohistogenic Sprague-Dawley rats. Animals younger than one year of age died approximately 1 to 2 weeks after the onset of the disease and represented approximately 17% of the total rat population. Clinically, the affected rats showed mainly spinal cord paralysis and enlargement of mandibular lymph nodes. Gross lesions were characterized by hepatosplenomegaly and enlargement of visceral lymph nodes. Histological examination of affected tissues and cells revealed generalized infiltration of blast cells accompanied by focal hemorrhages and necrosis. According to the FAB classification, it was concluded that ALL-SD corresponds to ALL type L2 in man. Therefore this leukemia of young rats may be an important model system for the study of human leukemia.