In this case report, a paraneoplastic syndrome caused by testicular neoplasia in a ten-year-old cryptorchid dog is described. Feminization and pancytopenia were observed, resulting from the testicular neoplastic production of estrogens. A diagnosis of testicular tumor and associated bone marrow suppression was made by ultrasonography and blood examination, with estrogen blood levels being severely elevated. Urinalysis revealed a urinary tract infection. Castration was performed together with a blood transfusion, and antibiotic treatment was started. After an initial improvement, the dog died suddenly after approximately three weeks. In this report, the importance is highlighted of identifying clinical signs associated with feminization in intact male dogs at an early stage, to avoid severe, potentially irreversible, hematological consequences due to bone marrow suppression. Elective orchidectomy of both testes is highly recommended in cryptorchid dogs as neoplastic transformation of the undescended testis may occur, with potentially fatal outcome.