To test the hypothesis that changes in cardiac index and mean arterial pressure (MAP) during and after a fluid bolus (FB) are altered by fluid temperature. Randomised, controlled, crossover trial. Research laboratory at Swedish teaching hospital. Twenty-one healthy adult volunteers. Subjects were randomly allocated to 500 mL of Ringer's acetate at room temperature (22°C; cold) or body temperature (38°C; warm). For 2 hours after starting the FB, we measured cardiac index, MAP, systolic blood pressure, diastolic blood pressure and pulse rate (PR) continuously. We recorded temperature and O2 saturation every 5 minutes during infusion and every 15 minutes thereafter. In a second session, volunteers crossed over. During the first 15 minutes, mean cardiac index increased more with warm FB (0.09 L/min/m2 [95% CI, 0.06-0.11] v 0.03 L/min/m2 [95% CI, 0.01-0.06]; P < 0.001). This effect was mediated by a significant difference in mean PR (+0.80 beats/min [95% CI, 0.47-1.13] v -1.33 beats/ min [95% CI, -1.66 to -1.01]; P < 0.010). In contrast, MAP increased more with cold FB (4.02 mmHg [95% CI, 3.63-4.41] v 0.60 mmHg [95% CI, 0.26-0.95]; P < 0.001). Cardiac index and MAP returned to baseline after a median of 45.3 min (interquartile range [IQR], 10.7-60.7 min) and 27.7 min (IQR, 5.3-105.0 min), respectively, after cold FB, and by 15.8 min (IQR, 3.8-64.3 min) and 22.7 min (IQR, 3.3-105.0 min), respectively, after warm FB. Intravenous FB at body temperature leads to a greater increase in cardiac index compared with room temperature, while the reverse applies to MAP. These findings imply that in healthy volunteers, when a room temperature FB is given, the temperature of the fluid rather than its volume accounts for most of the MAP increase. EudraCT no. 2016-002548-18 and Clinicaltrials.gov NCT03209271.