Acute lymphoblastic leukemia was diagnosed in 30 dogs. Diagnosis was confirmed by examination of blood smears and bone marrow aspirates. The most common clinical signs were lethargy, anorexia, splenomegaly, and pallor of mucous membranes. Three dogs were not treated, 3 dogs were treated with prednisone, and 3 dogs were treated with cyclophosphamide; survival times in these 9 dogs ranged from 1 to 60 days. Twenty-one dogs were treated with vincristine and prednisone; of these, complete remission was achieved in 4 dogs and partial remission was achieved in 4 dogs, with survival times ranging from 8 to 241 days.