We characterize the fairness of decentralized medium access control protocols based on CSMA/CA, such as IEEE 802.11, in large multi-hop wireless networks. In particular, we show that the widely observed unfairness of the protocol in small network topologies does not always persist in large topologies. This unfairness is essentially due to the unfair advantage of nodes at the border of the network, which have a restricted neighborhood and thus a higher probability to access the communication channel.