Oral lesions in twenty-nine immunocompromised patients were evaluated for the incidence of herpes simplex virus (HSV) infections during either cancer chemotherapy or cancer chemotherapy plus bone marrow transplantation (BMT). Patients' HSV antibody titers were not determined, and positive diagnoses were based solely on the results of viral cultures. Fourteen patients (48%) were found to have herpetic infections, which is comparable with incidence rates of 50% to 90% in antibody-positive patients and 40% to 50% in mixed antibody populations reported in the medical literature. However, this finding is in conflict with the 10.7% to 15.1% incidence rate cited in the dental literature for patients undergoing cancer chemotherapy. This underestimation is believed to reflect insensitivity in the criteria used for diagnosis in these studies.