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To enhance the throughput of a slotted ALOHA control channel in a radio communication system, we present and analyze a method for estimating the number of remote stations that are attempting to transmit to a central base station. Each of the contending remote stations randomly chooses to transmit with probability p. Our novel contribution is the use of information concerning the number of successful packet transmissions that arrive without retransmission (i.e., that are successfully received on their first transmission attempt), as a metric for accurately and robustly estimating the number of contending remote stations. This estimate is determined at the base station and then used to compute the optimal transmission probability p that is used as feedback to the remote stations for their use. The proposed estimation method is analyzed and shown to provide good steady-state performance for a variety of system models, including situations with noise and idling remote stations. The scheme is also shown to provide good tracking capabilities.