A Block Transmission and Acknowledgement (BTA) scheme, also called Block ACK, has been proposed in the IEEE 802.11e wireless LAN (WLAN) specification to improve efficiency of the medium access control layer. The idea of the BTA scheme is to transmit multiple data frames followed by only one acknowledgment frame in a transmission block. In this paper, we present a theoretical model to evaluate the saturation throughput for the BTA scheme under error channel conditions in the ad-hoc mode, validated with simulations. We show some advantages of BTA over the legacy MAC, and analyze how to select a proper number of frames for each transmission block. Results show that BTA is particularly effective in very high-speed wireless networks, and it is important that the number of frames in each block is negotiated before transmissions to provide better efficiency.