An analysis of event probabilities in wireless local area networks using distributed medium access control based on listen before talk is presented. More significantly it is shown how this can be used to calculate capacity or throughput for a given offered load for these networks. This in turn can be used to optimize system parameters for whatever performance aspect is of importance. The results are particularly applicable to systems designed for operation at 5 GHz. It is shown that for such systems with a usable system bandwidth of 70 Mbps in typical deployments the capacity or throughput are the order of 100 kbps/m2 for reasonable packet failure rates. Notes: G. Vanzini, contributed while working at HPLB on a student placement. Contact gvanzini@hotmail.com Notes: This abstract contains mathematical formulae which cannot be represented here. 25 Pages