To support mobility, mobile devices must be powered by batteries with limited energy. Thus, energy-efficient management becomes one of the most important design issues in mobile wireless networks. A well-designed energy-efficient MAC protocol for mobile wireless networks should be developed for both minimum energy consumption as well as maximum data throughput. IEEE 802.11 MAC protocol is based on the carrier sense multiple access with collision avoidance (CSMA/CA) medium access procedure to transmit/receive both control and data frames. We know that CSMA/CA wastes the scarce energy and bandwidth due to frame collisions and lengthens the transmission delay due to waiting backoff time, especially in heavy traffic load. In addition, the IEEE 802.11 power management scheme does not specify how to determine the ATIM window size in a beacon interval. The fixed ATIM window size can not always accommodate the dynamically changing traffic conditions. To conquer these problems, we propose a novel power saving MAC protocol for IEEE 802.11 ad hoc networks. The proposed protocol possesses two properties: (1) based on overhearing ATIM frames in an ATIM window, power saving (PS) stations can be scheduled to transmit data frames in order after the ATIM window, and (2) depending on the fluctuant traffic conditions, PS stations adjust the ATIM window size dynamically to save energy and achieve the superior network throughput. Through extensive simulations, we demonstrate the advantages of our new MAC protocol.