A 13-year-old mixed-breed dog showing ascites, anorexia and anemia was found to have leukocytosis with marked eosinophilia, splenomegaly and hepatomegaly. The dog died 4 days after initial presentation and was diagnosed with splenic high-grade B-cell lymphoma at necropsy. Remarkable infiltrations of eosinophils were observed in spleen and liver tissues. The eosinophilia and infiltration of eosinophils into the lesions could have been associated with B-cell lymphoma because causes other than lymphoma were excluded. This is the first report of eosinophilia and eosinophilic infiltrations into neoplastic lesions in a dog with high-grade B-cell lymphoma.