Distributed medium access control (MAC) is important for efficient exploitation of spectrum opportunities in cognitive radio networks. This letter proposes a channel-aware distributed MAC protocol for multichannel cognitive radio networks. The transmitting nodes use channel state information to exploit the available multiuser and multichannel diversity, by contending only under relatively favorable channel conditions. Towards this end, each node utilizes a threshold that is dynamically updated according to certain rules. Simulation results corroborate the throughput gains achieved by the proposed protocol.