The aim of this study was to determine if evolution of fever after administration of indomethacin to febrile patients could separate those with fever of infectious origin from those with non-infectious origin. All patients with a rectal body temperature superior than 38 degrees C for at last 5 days and without any antibiotic or antipyretic therapy for more than 48 hours, were included in a 1 year prospective study. Each patient received one time 50 mg of indomethacin and rectal body temperature was obtained every 3 hours for 12 hours. Forty-five patients were included, 20 in group I (fever of infectious origin) and 25 in group II (fever of non-infectious origin). The mean age and mean initial body temperature were similar in the two groups. After administration of indomethacin, mean duration and mean amplitude of abatement of fever were similar in the two groups. Hypothermia was observed more frequently in group II (28%) than in group I (10%) (p < 0,05) and was preferentially associated to a malignant lymphoma. Also, diagnostic procedure with indomethacin is of little interest to separate fever of infectious origin from other, but hypothermia would suggest a fever due to a malignant lymphoma.