A study was conducted to evaluate the ability of a commercially available automated estrus detection (AED) system to detect cows in estrus. Estrus was synchronized in lactating dairy cows (n = 216) fitted with an ear-attached sensor that generated estrus alerts based on a combination of physical activity and rumination time data. For seven days after induction of luteolysis, cows were monitored for estrus behaviors using a combination of visual observation and automated monitoring of standing events. Ovulation time was determined via transrectal ultrasonography. The sensitivity, specificity, negative predictive value, positive predictive value and balanced accuracy of the AED system estrus alerts compared with the reference test were 92%, 69%, 53%, 96%, and 80%, respectively. Average alert duration was 13.5 h and interval from alert generation to ovulation was 23.8 h. Parity, milk production level, and circulating progesterone concentrations before estrus were all associated with significant changes in estrus expression.