Histiocytic sarcoma is a malignant neoplastic proliferation of atypical histiocytes with tendency to spread, characterized by fast progression to disseminated form--disseminated histiocytic sarcoma. Cytopathology is a low, invasive, cheap, and quick method of diagnosis commonly used in veterinary oncology. The aim of the presented study was description of cases of visceral histiocytic sarcomas in dogs diagnosed by cytopathology and immunocytochemistry. The study was conducted on 5 dogs which were brought to the veterinary clinic because of unspecific clinical signs and tumoral masses recognized in the thoracic or abdominal cavity. Samples of cells were collected during ultrasonography-assisted fine-needle aspiration biopsy (FNAB), smears were stained with Giemsa method and immunocytochemistry (CD3, CD79alpha, cytokeratin, vimentin, desmin) was also performed in all patients. Four of five dogs were Bernese mountain dogs, nonspecific clinical signs of systemic disease were present in all cases. Visceral mass or masses were detected by ultrasonography or radiography. Final diagnosis of histiocytic sarcoma was obtained on the basis of routine cytopathological examination and confirmed by immunocytochemistry. On the basis the results obtained it can be stated that in cases of typical clinical and cytopathologic pictures, examination of cellular samples collected during ultrasonography-assisted fine-needle biopsy supported by some immunocytopathological characteristics seems to be sufficient method of diagnosis of histiocytic sarcoma in dogs. Visceral histiocytic sarcoma should be included into differential diagnosis in every Bernese mountain dog with nonspecific clinical signs, ambiguous results of hematologic examination and when tumoral mass or masses within a body cavity were detected in imaging techniques.