SummaryInfection with Herpesvirus hominis, often associated with oral ulceration, was found to be more frequent in patients with myeloproliferative and lymphoproliferative disorders than in normal populations or patients with other diseases. This increased frequency was not associated with any deficiency of the humoral antibody response, suggesting a possible impairment of cellmediated immunity. The otherwise untreatable oral lesions appeared to respond effectively to local irradiation. IntroductionThe frequent occurrence of severe oral ulceration in patients suffering from various types of leukaemia and lymphoma prompted an investigation into the aetiology of the oral lesions. Clinical impressions suggested an herpetic origin and others have commented on the frequency of herpetic infections in patients with myeloproliferative and lymphoproliferative disorders (Stewart, 1950;Ultman et al., 1959;Muller et al., 1972) few patients, however, have been subjected to virological investigations. In the present study virological investigations were used to establish the aetiological role of Herpesvirus