Summary Objective To investigate the effect of fever episodes and fever characteristics on heart rate, arterial blood pressure and arterial oxygen saturation of intensive care unit (ICU) patients. Methods This was a prospective study conducted in the medical–surgical ICU of General University Hospital of Patras, Greece. All patients who were consecutively admitted from October 2005 to February 2006 and manifested fever during ICU stay were enrolled. A tympanic membrane or an axillary thermometer was used for the measurement of patient temperature. Standard monitoring parameters were recorded by nursing personnel at 1-h intervals. Results Seventy-five ICU patients manifested fever during the study period. Increase of core temperature during fever episodes was followed by a significant increase in heart rate ( p < 0.001) and decreases in arterial blood pressure ( p < 0.001) and arterial oxygen saturation ( p = 0.002). Alterations of heart rate and arterial blood pressure were significantly affected by magnitude of fever, while alteration of arterial oxygen saturation was affected by etiology of fever. Conclusions The present findings confirmed the effect of fever episodes on standard monitoring parameters of ICU patients. However, alterations of these parameters, although statistically significant, were not clinically important and cannot guide antipyretic treatment.