BACKGROUND: Digital rectal thermometers typically have 2 distinct modes of operation: dwell or monitor, in which body temperature is displayed continuously, and predictive, in which body temperature is mathematically estimated on the basis of the curve of temperature rise that occurs after the thermometer is inserted. Although the dwell mode is generally considered more accurate, the predictive mode allows more rapid measurement. Direct comparisons of these 2 methods are lacking. OBJECTIVE: To determine if measurements of rectal temperatures obtained with the predictive mode differ significantly from those obtained with the dwell mode. METHODS: For each of 10 subjects, both dwell and predictive modes were used to measure early morning rectal temperature. For comparison of the 2 modes during states of temperature flux, 8 subjects had rectal temperatures measured after swimming in cold water. Initial predictive values were measured directly after each subject exited from the water. Initial dwell values were estimated by linear extrapolation of measured 3- and 6-minute dwell values. Paired-sample statistical methods were used to compare data. RESULTS: Measurements of early-morning temperatures obtained with the predictive mode did not differ significantly from those obtained with the dwell mode (P = .64). Predictive values also did not differ significantly from extrapolated dwell values for measurements obtained during mild temperature flux after immersion in cold water (P = .33). CONCLUSION: The predictive and dwell modes of digital rectal thermometers yield similar measurements of temperature during steady-state conditions and during states of mild temperature flux.