In the recent financial crisis we saw liquidity in the stock market drying up as a precursor to the crisis in the real economy. We show that such effects are not new; in fact, we find a strong relation between stock market liquidity and the business cycle. We also show that investors' portfolio compositions change with the business cycle and that investor participation is related to market liquidity. This suggests that systematic liquidity variation is related to a “flight to quality” during economic downturns. Overall, our results provide a new explanation for the observed commonality in liquidity.