In IEEE 802.11 wireless LANs, the DCF access method and the PCF access method operate alternatively within a superframe to service the time-varying traffic demands. Due to different medium access mechanisms deployed by DCF and PCF, they work well for some particular types of traffic scenarios respectively, while their performance may degrade under some other types of traffic demands. None of them can keep high performance under all types of traffic demands. In this paper, we propose an adaptive control algorithm to keep the aggregate throughput of access points close to the best capacity of both DCF mode and PCF mode upon various traffic requests, by dynamically adjusting the duration of DCF mode and PCF mode within a superframe based on recent traffic scenarios in wireless LANs. We modified the MAC module of IEEE 802.11 wireless LANs in OPNET to support our algorithm. Extensive simulations have been done using OPNET. The simulation results show that high performance of access points is achieved during the dynamic traffic demands. The new control algorithm only requires simple extensions to the MAC layer of access points and is compatible with the MAC layer defined in IEEE 802.11 standard.