In IEEE 802.11[1] wireless LANs based on CSMA/CA (Carrier Sense Multiple Access with Collision Avoidance), parameters such as contention window (CW) significantly affects its throughput performance. In this paper, we propose a novel CW control scheme in order to achieve the high throughput performance in dense user environments. While the standard CSMA/CA mechanism employs an adaptive CW control according to the number of packet retransmissions, the proposed scheme uses the optimum CW size, which is a function of the number of terminal stations (STAs). In the proposed scheme, an access points (AP) estimates the number of STAs from the measured packet collision probability, and derives the optimum CW size based on a theoretical analysis using a Markov chain model. With simulation experiments in a dense environment, we evaluate the performance of the proposed scheme and show that it significantly improves the throughput performance.