Next generation wireless local area networks (WLANs) such as the IEEE 802.11n standard are expected to rely on multiple antennas at both transmitter and receiver to increase throughput and link reliability. However, these improvements come at a significant increase in signal processing and hence hardware complexity compared to existing single-antenna systems. This paper presents, to the best of the authors' knowledge, the first 4 x 4 MIMO-OFDM WLAN physical layer ASIC based on the OFDM specifications of the IEEE 802.11a standard. The ASIC achieves an uncoded throughput of 192 Mbps in a 20 MHz channel resulting in a spectral efficiency of 9.6 bits/s/Hz. We describe the hardware architectural differences to single-antenna OFDM systems as well as the extensions made necessary by the use of multiple antennas. Our implementation provides reference for the silicon complexity of MIMO-OFDM systems.