Objective This study determined the effect of urine flow rate on bladder temperature in critically ill adults. Design The design was pretest-posttest quasi-experimental. Setting The study took place at a tertiary care center in western Washington. Patients Convenience sampling resulted in an intervention group of 35 and a control group of 25 patients who had undergone cardiac surgery. Intervention A diuretic, administered intravenously as part of usual postoperative care, provided variation in the urine flow rate. Methods Urinary bladder temperature (UBT), pulmonary artery temperature (PAT), and urine flow rate data were collected at 2-minute intervals for 60 minutes preintervention and 60 minutes postintervention. Results Unlike the control group, who experienced no significant change in mean urine flow rate or gradient (0.00°C) from prephase to postphase, the intervention group experienced both a significant and close to 10-fold increase in urine flow rate ( P < .001) and a significant decrease (0.09°C) in mean UBT-PAT gradient ( P < .001). The change in gradient experienced by the intervention group compared with the control group was the result of a net heat loss of 0.05°C in UBT and a net heat gain of 0.04°C in PAT. Conclusions Although statistically significant, the mean change in UBT-PAT gradient between the intervention and control groups was not clinically important, indicating that bladder temperature remains reliable even with significant changes in urine flow rate.