The MAC-hs protocol of HSDPA ensures retransmissions of erroneous blocks and in-sequence data delivery to the upper layer. The first function is fulfilled through the HARQ mechanism. The second function is achieved by handling sliding transmitting/receiving window and by numbering of blocks. In this paper, the MAC-hs performance for different MAC-hs window sizes and channel conditions are studied. Simulations show that the parameter setting has to be advisably set up in order to prevent deterioration of the protocol performance.