MIMO techniques using multiple antennas for both transmitting and receiving have recently manifested themselves to be very promising for future broadband wireless networks. Aiming to leverage the impact of these MIMO techniques on network protocol design in wireless LANs (WLANs), we take a utility approach to study channel-aware weighted proportional fair medium access control (WPFMAC)for QoS provisioning. Simply put, in this utility based approach, every user in a WLAN attempts to maximize its own utility, and the optimization procedure takes place in two stages: a channel contention phase that arbitrates fair channel access across the users via combining adaptive persistence mechanism with random backoff, and a data transmission phase that determines the transmission duration based on the channel conditions in each transmission round. Furthermore, adaptive beamforming is carried out by using the training signals embedded in the RTS/CTS handshake to enhance the spectral efficiency of the MIMO links. Using a stochastic approximation method, together with the Lyapunov stability theorems, we establish the stability of the adaptive persistence mechanism in the proposed WPF MAC and analyze the fairness across the users therein.