The next generation of 802.11 WLANs is expected to support Multi-User MIMO (MUMIMO) transmission, whereby parallel transmissions can be simultaneously made to multiple stations. This significantly extends the MIMO support introduced by the 802.11n standard and is, for example, included as part of the current draft 802.11ac standard that aims to support wireless data rates in excess of 1 Gbps. MU-MIMO offers much greater flexibility in scheduling MIMO transmissions, but immediately raises the question of how best to allocate MIMO spatial streams amongst network flows so as to balance fairness and performance. In this paper, the authors consider the proportional fair allocation in an 802.11 WLAN that supports Multi-User MIMO (MU-MIMO) transmission by one or more stations.