Current automated technologies for measuring calf body temperature are limited in their accuracy and practicality in commercial settings. We hypothesized that oral temperature might be a suitable method for body temperature estimation for automated health monitoring. The objective of this study was to validate oral temperature as an alternative to rectal temperature in preweaning dairy calves, particularly for detecting a fever. One hundred fifty calves were followed from 1 to 28 days of life, having oral and rectal temperatures taken every other day. Oral temperature was typically slightly lower than rectal temperature but reliably reflected systemic changes in body temperature and showed very good diagnostic accuracy at fever detection with a sensitivity of 79% and a specificity of 98%. AUC = area under the curve.