The wireless mesh networks have been a very promising solution for providing last few miles wireless connectivity in the metropolitan area networks. A new medium access control scheme is proposed to improve throughput in wireless mesh networks. The proposed scheme is to adjust the minimum size of contention window of mesh clients according to the information of transmission queue at relay nodes. The relay node carries the load information on an ACK frame by using the reserved subtype fields. The proposed scheme gives a priority to the relay node for transmitting packets when the relay node becomes a bottleneck. The simulation results show much better performance from the proposed scheme in comparison with that from original CSMA/CA scheme.