The antipyretic effect of diclofenac sodium 0.2 mg/kg i.v. was studied prospectively in 10 ICU patients. Patients with renal failure and hypovolaemia were excluded from the study; mean basal temperature (measured by the pulmonary artery thermistor) was 38.92°C±0.413 SD. In 9 of the 10 patients, the temperature fell by more than 0.5°C within 1 h of administration of the drug. A minimum mean of 37.80°C±0.636 SD was obtained by the hour 3; the temperature then remained lower than basal throughout the entire observation period (6 h). Changes in haemodynamics and oxygen consumption were consistent with the reduction in temperature. Changes in renal function were transient and did not require any therapeutic intervention. We conclude that the proposed dosage (in the selected patient population) constitutes effective antipyretic treatment devoid of major side effects.