Herpesvirus saimiri resulted in the development of malignant lymphoma in 2 of 6 spider monkeys (Ateles geoffroyi) inoculated via the intramuscular route. One monkey died 129 days after inoculation and the other was sacrificed at 179 days. The disease resembled H. saimiri induced lymphoma as seen in marmosets (Saguinus oedipits) and owl monkeys (Aotus trivirgatus) ; characterized by extensive lymphoblastic infiltration in multiple organ systems. In one monkey solid tumor masses were present. Two other animals had lymphocytic hyperplasia and 2 died without histopathological lesions. The findings expand the already wide host spectrum of this oncogenic herpesvirus.