Purpose: To investigate the accuracy, precision and validity of fever detection of tympanic membrane (TM), temporal artery (TA) and axillary temperature (AT) compared with pulmonary artery temperature (PA). Methods: Repeated-measures design was conducted for one year on 83 adult cardiac care unit patients with pulmonary artery catheters after open heart surgery. Sequential temperature measurements were taken three times at 20-minute intervals. Accuracy, precision, repeatability, and validity of fever detection were analyzed. Results: Mean pulmonary artery temperature was $37.04^{circ}C$ (SD $0.70^{circ}C$). The mean (SD) offsets from PA, with the mean reflecting accuracy and SD reflecting precision, were $-1.31^{circ}C$ ($0.75^{circ}C$) for TA, $-0.20^{circ}C$ ($0.24^{circ}C$) for TM, and $-0.97^{circ}C$ ($0.64^{circ}C$) for AT. Percentage of pairs with differences within ${pm}0.5^{circ}C$ was 9.6% for TA, 19.7% for AT, and 91.6% for TM. Repeated measurements with all three methods had mean SD values within $0.04^{circ}C$. Sensitivity, specificity, and positive and negative predictive values of tympanic measurements were 0.76, 1.0, and 1.0, and 0.90, respectively. Conclusion: Results show that TM best reflects PA, and is most consistent, accurate, and precise. AT tends to underestimate PA, and TA is least accurate and precise. Therefore tympanic membrane measurement is a reliable alternative to other non-invasive methods of measuring temperatures.