We study optimal admission control of secondary users (SUs) in cognitive radio (CR) networks in presence of preemption. In this model, when a primary user (PU) arrives to the system and finds all the channels busy, it preempts an SU unless all the users in the system are PUs. We apply admission control on the SUs only. Using dynamic programming (DP), we find the optimal admission control policy that maximizes the long- run average profit. As our main contribution, we show that the optimal admission control of the SUs depends only on the total number of users in the system (i.e. it does not depend on the number of PUs and SUs in the system individually) and is of threshold type. Therefore, although the system is modeled as a two-dimensional Markov chain, our findings allow simple and efficient computa- tion of the optimal control policy.