33 children with acute lymphatic leukemia and 33 healthy controls were longitudinally studied for herpesvirus infections. Active herpes simplex-virus, varicella-zoster virus and cytomegalovirus (CMV) infections were more frequent in patients than in controls. CMV and Epstein-Barr virus infections were often inapparent or associated with infections of the upper respiratory tract. Analysis of serological datas revealed a coincidence of active CVM infection and lethal course of the leukemia. This may be a result of the immunodeficiency in leukemia patients caused by disease and therapy. An additional influence of the immunosuppressive effect of active CMV infections on the course of the disease is discussed.