Fifty-seven episodes of anorectal infection in 44 patients with malignant diseases primarily leukemia or lymphoma, have been retrospectively reviewed. Seventeen patients died in hospital, but only in seven cases was the anorectal infection a major contributing cause of death. The most important prognostic indicator of outcome was number of days of neutropenia during the infectious episode. Cultures obtained at the time of surgical drainage or by needle aspiration of the wound revealed multiple organisms in 26 of 29 instances, and anaerobic organisms were the commonest isolates. Anorectal infection was controlled in 28 (55%) of 51 treatment courses when antibiotics were the only treatment given. However, if the antibiotic regimen included both an aminoglycoside and an antibiotic with anaerobic coverage, control of infection was observed in 15 (88%) of 17 cases. There were 26 surgical procedures performed, with acceptable morbidity. Infection was controlled in 19 (73%) of 26 cases treated with surgery and antibiotics. The results support managing most of these infections initially with medical treatment, using an antibiotic regimen that includes an aminoglycoside and a specific drug against anaerobes. Surgery is recommended if there is obvious fluctuance, a significant amount of necrotic tissue evident, or progression of the infection locally or continued sepsis after an adequate antibiotic trial.