BACKGROUND: Fever in patients can provide an important clue to the etiology of a patient's symptoms. Non-invasive temperature sites (oral, axillary, temporal) may be insensitive due to a variety of factors. This has not been well studied in adult emergency department patients. To determine whether emergency department triage temperatures detected fever adequately when compared to a rectal temperature. METHODS: A retrospective chart review was made of 27130 adult patients in a high volume, urban emergency department over an eight-year period who received first a non-rectal triage temperature and then a subsequent rectal temperature. RESULTS: The mean difference in temperatures between the initial temperature and the rectal temperature was 1.3 °F (P<0.001), with 25.9% of the patients having higher rectal temperatures ≥2 °F, and 5.0% having higher rectal temperatures ≥4 °F. The mean difference among the patients who received oral, axillary, and temporal temperatures was 1.2 °F (P<0.001), 1.8 °F (P<0.001), and 1.2 °F (P<0.001) respectively. About 18.1% of the patients were initially afebrile and found to be febrile by rectal temperature, with an average difference of 2.5 °F (P<0.001). These patients had a higher rate of admission (61.4%, P<0.005), and were more likely to be admitted to the hospital for a higher level of care, such as an intensive care unit, when compared with the full cohort (12.5% vs. 5.8%, P<0.005). CONCLUSIONS: There are significant differences between rectal temperatures and non-invasive triage temperatures in this emergency department cohort. In almost one in five patients, fever was missed by triage temperature.