We examine the impact of changes in equity-option margin requirements on the liquidity of options and underlying stock markets. We find that the decrease in margin was associated with an increase in spreads and trade informativeness, and a decrease in depth for the underlying stocks. In contrast, option spreads decreased indicating a change in the relative allocation of informed traders between the two markets. When the required margin was increased, no significant change was observed in the underlying stocks, but option spreads increased. Overall, our results indicate that uninformed traders are more sensitive to the margin dimension of trading costs.