Tuberculosis (TB) remains a serious health problem in many developing countries. As impaired immunity is closely associated with the pathogenesis of TB, an increase in the incidence of this disease among patients with cancer is anticipated. This retrospective analysis of children from an area of high TB prevalence was done to determine the incidence of TB in children treated for cancer. Of 390 children admitted between 1983 and 1990, 278 were evaluated. Standardized morbidity ratios showed that children with malignancies had an 11-times greater chance of developing TB than children of the same age group in the general population. The highest incidence occurred in children with Wilms' tumour (5 of 41). In the majority of patients (77%), TB was diagnosed during the initial period of hospitalization and chest X-ray suggested post-primary TB. This suggests reactivation of quiescent mycobacteria during a period of impaired immunity. Children from an area with a high incidence of TB who develop malignancy should be actively investigated for tuberculosis prior to the start of immunosuppressive therapy. We speculate that these children would benefit from a course of preventive anti-tuberculosis treatment at the start of chemotherapy.