In wireless local area networks (WLANs), the demand of the multimedia applications such as voice telephony and peer-to-peer content distribution has followed the necessity of quality-of-service (QoS) control for uplink flows. However, in uplink WLANs, the hidden-station problem causes difficulties in the QoS control because of unfair collision probability. In this paper, we point out this hidden-station problem and clarify the following unfairness between user datagram protocol (UDP) and transmission control protocol (TCP) uplink flows: 1) the effect of collision caused by hidden-station relationship on throughput and 2) the instability of the throughput depending on the number of hidden stations. To solve these problems, we propose a mechanism that first groups stations according to the hidden-station relationship and type of transport protocol they use then assigns a transmission permitted period to each group. Our proposed mechanism eliminates the collisions between hidden stations and provides a flexible control over bandwidth allocation to UDP and TCP. Its performance is shown through simulation.