Fever is frequently a symptom in patients suffering from cancer and in most cases it is related to infections or complications of the treatment. Some cancers can also be the direct cause of fever. A total of 28 episodes of fever in 8 patients with cancer were studied. The diagnoses were: 3 patients with lung cancer, 1 patient with chronic myelogenous leukemia, 1 patient with kidney cancer, 2 patients with non-Hodgkin lymphoma, and 1 patient with Hodgkin's disease. Were included cancer diagnosed patients of any age and sex, with three or more episodes of fever of more than 37.5 C (with a case of 38.5 C or more) after having eliminated any infectious etiology or fever caused by drugs. Were not excluded any patients who had received whole blood or blood derivative transfusions, chemotherapy or antibiotic treatment up to 48 hs before the fever peak. The patients were given intramuscularly 500 mg of aspirin or 50 mg of indomethacin. The first response in the temperature curve was evaluated while checking the axillary temperature six hours after drug administration and 48 hs later if the fever persisted. The patients who were given aspirin at first, were then given indomethacin and vice versa, using the same criteria to evaluate the response. In patients treated with indomethacin the temperature diminished quickly and completely (Fig. 1), unlike the effect achieved with the use of aspirin (Fig. 2). All patients treated with indomethacin also showed a remarkable clinical improvement which was not observed when aspirin was used.(ABSTRACT TRUNCATED AT 250 WORDS)