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Pilot symbols reduce the transmit energy for data symbols per orthogonal frequency division multiplexing (OFDM) symbol under a fixed total transmit power constraint, while they facilitate channel estimation. In this paper, we investigate the effect of the pilot-to-data power ratio (PDPR) on symbol error rate (SER) and spectral efficiency of multiple-input multiple- output (MIMO) OFDM systems with adaptive modulation and a linear receiver. We derive the optimal PDPR to minimize the average SER of multilevel quadrature amplitude modulation (M- QAM) MIMO-OFDM systems. Wishart matrix analysis can be used to analyze the effect of channel estimation error on the average SER performance of the M-QAM spatial multiplexing MIMO-OFDM systems with zero-forcing (ZF) receiver. By using the optimal PDPR in the MIMO-OFDM system with adaptive modulation, about 2.5dB gain can be obtained. It is quite practical to use the results to design the PDPR for typical cases, since there is a fairly broad range of PDPR which minimizes the SER.