Concern about energy-efficient medium access control (MAC) protocols for wearable devices is increasing owing to support for healthcare services using wireless body sensor networks (WBSNs). The most popular energy-efficient MAC protocol for WBSNs is the IEEE 802.15.6 standard, which adopts carrier sensing multiple access with collision avoidance (CSMA/CA). The CSMA/CA mechanism of the IEEE 802.15.6 standard allows differentiated channel access by assigning a different size of contention window to the nodes, each of which has a different priority. However, the existing CSMA/CA of IEEE 802.15.6 still cannot guarantee successful data transmission in error-prone channels and congested network environments, which leads to wasted energy owing to data retransmission. In this paper, we propose an adaptive priority-based MAC (AP-MAC) protocol for IEEE 802.15.6 WBSNs, which utilizes transmission opportunities suitable for WBSNs. For this, data types are classified with predefined priorities with each data type having a different opportunity to access the channel. In addition, the priority of each classification is updated adaptively according to the update metrics channel state and congestion level. Simulation results show the enhanced performance of the AP-MAC protocol compared with that of the IEEE 802.15.6 standard.