During six-month period, 102 consecutive episodes of fever in 68 children (ranging from 1 month to 14 years of age) with malignant diseases were prospectively evaluated. Sixty-five had acute lymphoblastic leukemia, nine had acute myeloblastic leukemia, nine had malignant lymphoma (four Hodgkin and five non-Hodgkin), five had chronic myeloid leukemia, four had rhabdomyosarcoma, three had CNS tumors, two had neuroblastoma, one had Wilms, and four had other malignant tumors. Forty cases (39.2%) showed severe neutropenia (500 neutrophil/m3) during the episode. S. aureus, E. coli, and S. pyogenes were in 53% of the 75 microbiologic isolates. Twenty-two percent of the viral studies were positive. Mycologic studies were all negative, except one case with C. Albicans. Pneumonia (33 cases), cellulitis (15 cases), pharyngitis (12 cases), and varicella (11 cases) were the most common final diagnosis. Seventy-one percent of the episodes were etiologically documented (by bacterial isolate, characteristic serology, and/or typical clinic picture); 19% of the febrile episodes were probable infections, and 10% were fever of uncertain cause. Ninety percent of the cases responded well to therapy, and mortality of this series was 7%. Gentamicin, Carbenicillin, and Methicilin were the more common antibiotics employed. We conclude that in our population 1) infection is a frequent cause of morbidity in children with malignant diseases; 2) the most common cause of the febrile episodes is bacterial infection; 3) S. aureus, E. coli and S. pyrogenes are the most frequent bacterial isolates, and P. aeruginosa is infrequent; 4)viral infections are relatively frequent in this group of children; and 5) with adequate management, the mortality is low.