In this paper, we propose a Medium Access Control (MAC) extension, called Medium Access for Contention Assistance (MACAT), to enhance Quality-of-Service (QoS) for IEEE 802.11 and IEEE 802.11e co-existing networks. In MACAT, IEEE 802.11e stations act as assistants for channel access of legacy stations with real-time traffic. With the assistance of 802.11e stations, the legacy stations have a higher probability to deliver their real-time data. The performance of our MACAT protocol is investigated through the NS-2 simulator. The experimental results indicate that in terms of bitrate, delay, and packet loss rate, the performance of the legacy stations with real-time traffic is upgraded by our MACAT while the QoS of IEEE 802.11e stations can be maintained.