Accurate temperature measurement is critical to the assessment and management of temperature fluctuation in the acutely ill adult. Unfortunately, an accurate, noninvasive method to measure core temperature has yet to be established, and current instruments produce a wide range of temperatures for any given patient. This article provides an integrative review of studies comparing selected invasive and noninvasive temperature measurement methods in acutely ill, hospitalized adult patients. Medline and CINAHL databases were searched to locate published studies on temperature measurement in the adult hospitalized patient. A partial list of primary search terms included core temperature measurement, oral temperature measurement, temporal artery thermometry, and tympanic thermometers. Studies that were data based and included comparison of a tympanic, temporal artery, and/or oral noninvasive temperature measurement to a pulmonary artery or esophageal invasive core measurement in an adult population were included in the analysis. The search method produced 223 publications and abstracts for initial review; 23 (10%) met the inclusion criteria. The only study evaluating the use of temporal artery thermometry in the adult population found the instrument to be unreliable. Results also indicate that high-quality evidence supporting the accuracy of tympanic thermometry, the preferred instrument for noninvasive core temperature measurement in many acute care settings, is lacking, and in fact, the most recent high-quality studies evaluating the accuracy of this instrument fail to show support for its use. Evidence does, however, support the use of oral thermometry as an accurate means of temperature assessment in the adult, acutely ill population.