A quarterly time series of the aggregate commission rate of NYSE trading for the period 1980-2003 is developed. The aggregate commission rate is of significant size, captures trading cost, and reflects market illiquidity. Consistent with financial theory, I find a positive relation between market returns and the aggregate commission rate. The impact of the aggregate commission rate on market returns survives a number of robustness checks and is significant after controlling for interest-rate factors, trading volume, and the variability of trading volume. Overall, the findings suggest that market-wide liquidity is a state variable important for asset pricing.