In a prospective study of 44 patients, who underwent autologous bone marrow transplantation (ABMT) for solid tumours, the results of serial viral cultures and determination of complement-fixing antibodies to herpes simplex virus (HSV) were analysed. No primary infections with HSV occurred. Of 31 initially HSV-seropositive ABMT patients, 61% developed positive HSV cultures. Half of these patients showed a significant antibody rise. Active HSV infection did not influence the recovery of the bone marrow after ABMT, but coincided with a significantly increased number of febrile days.