This study finds that aggressive tax strategies adopted by a firm affect idiosyncratic stock return volatility. Aggressive tax strategies, which I measure as tax paid by a firm divided by pretax income (adjusted for special items), are associated with higher levels of idiosyncratic stock volatility. Uncertainty associated with tax strategies may result due to several factors, such as penalties, fines, and additional tax payments if particular tax strategies are disallowed by taxation authorities, or if there are changes in tax rules. Such uncertainty affects the future cash flows of a firm and is reflected in more volatile stock returns. Financial constraints, corporate governance mechanisms, and information environments surrounding a firm influence the relation between idiosyncratic volatility and effective tax rates.