Hematological malignancies are among the most commonly occurring neoplasms of domestic animals. The viral cause of lymphosarcoma in birds and cats is established and is suspected in a number of other species. Six patients with various hematological malignancies owned pets in which neoplasms developed. Five of the pets had lymphosarcoma and one had multiple myeloma. Because of the frequency of these malignancies in humans and lower animals it is most probable that the neoplasms co-occurred in patient and pet because of chance factors. Whether this is an epidemiologically significant relationship remains to be determined.