All children with leukaemia or malignant tumour receiving cytostatic agents were followed by virological tests over three years. Serological tests or virus isolation in 200 children demonstrated 238 instances of viral infection. Only those of the herpes group led to severe symptoms which, however, responded to treatment. The course of infections induced by respiratory viruses was no different from that in children not receiving immunosuppressive agents. It was possible, by using a "virus risk register", to give early protection to infected children by the appropriate prophylactic means. In addition to the virus infections there were seven cases of Pneumocystis carinii pneumonia which ended fatally in two. Since the prophylactic use of cotrimoxazole there have been no further cases.