Livestock farming technologies have a tremendous potential to improve and support farmers in herd management decisions, in particular in reproductive management. Nowadays, estrus detection in cows is challenging and many detection tools are available. The company Smartbow (Weibern, Austria) developed a novel ear-tag sensor, which consists of a 3D-accelerometer that records head and ear movements of cows as basis for algorithm development and further analyses. Estrus detection by the SMARTBOW system is primarily based on an increased activity combined with behavioral changes. In this study, the system was installed on a commercial dairy farm in Slovakia and Holstein-Friesian cows were equipped with SMARTBOW Eartag sensors. Exceeding cow specific thresholds for activity and behavioral changes, an estrus alert was generated. Retrospectively, reproductive performance data were used to evaluate the accuracy of estrus alerts generated by the SMARTBOW system. Sensitivity, specificity, positive and negative predictive value, accuracy, and error rate for detecting estruses were 97%, 96%, 98%, 94%, 96%, and 2%, respectively. In summary, the SMARTBOW system is suitable for an automatic estrus detection of estrus events in indoor housed dairy cows.