Recently, cooperative communications have been proposed to improve the performance of wireless networks. In this paper, we develop a medium access control (MAC) protocol for cooperative wireless networks, which is ready to be incorporated into IEEE 802.11 networks. In the proposed MAC protocol, when a retransmission is needed, the packet is retransmitted from a cooperating node, that is able to overhear the transmitted signals from the source and decode the information bits correctly, after re-encoding the information bits using a properly chosen error correcting code. To ensure orderly cooperation among multiple cooperating nodes using a minimum signaling overheard, a deterministic backoff scheme is employed to specify the order that the source and cooperating nodes access the channel. Our simulation results demonstrate that node cooperation can increase system throughput and improve network coverage.