A histiocytoma is a benign skin tumor found in dogs. The tumor can develop quickly, they are made up of rapidly developing cells and usually occur in dogs under 5 years of age. Histiocytomas vary in size but are usually raised and furless. They are most commonly found on the head, neck, ears, and limbs. Cytology can help to diagnose a histiocytoma. The veterinarian will perform a fine needle aspirate and examine the sample with a microscope. Most histiocytomas will go away on their own within two or three months. Surgical removal may be necessary if the tumor does not regress or if it grows rapidly to a large size. It is always a good idea to have your veterinarian check out any new lump that is found on your pet.