The viruses isolated during infective episodes over a 5-year period of 93 children with acute lymphoblastic leukemia (ALL) and 107 children with other malignancies have been studied. Viruses were identified by the fluorescent antibody technique (FAT), electron microscopy, and culture. Viruses were isolated on 204 occasions. Rhinovirus and RSV occurred significantly more often in the patients with ALL, and adenovirus more commonly in the solid tumor patients. Viruses were responsible for 4 of the 69 deaths. Multiple different virus isolation during the same infective episode was significantly more common than in a control group. Where paired sera were available, only 10 out of 27 children showed at least a fourfold rise to the isolated virus. A rise in complement-fixing antibody is confirmed as a very unreliable method of detecting virus infection in immunosuppressed children. Viruses are important pathogens in children with cancer and assume greater relevance now that over 60% of malignant disease in childhood can be cured.