Motivated by the inefficient usage of the licensed spectrum across many frequency bands, cognitive radio (CR) was proposed to promote the spectrum utilization by opportunistically exploiting the existence of spectrum "holes." Under this new paradigm, cognitive (unlicensed) user is allowed unobtrusive access to a channel exhibiting negligible primary (licensed) user activity. Cooperative communication is increasingly regarded as a way to address hidden terminal issues and to improve spatial diversity for the same spectrum frequency band. In this paper, we investigate a cooperative transmission of primary traffic by cognitive users (cognitive relaying). The scenario focuses on one cognitive transmitter which has the option to relay traffic of the primary assuming that the primary is oblivious to its presence and thus exclude the possibility of spectrum leasing. To demonstrate the feasibility and performance of this cooperative relay for CR, numerical analysis based on information theoretic approach is used in order to discuss the advantage and capacity limits of the strategy.