This study tests empirically the hypothesis that corporate income taxes are neutral for firm efficiency. We exploit the fact that the tax definition of cost does not overlap fully with an accounting definition of cost and develop an instrument for taxation which relies on exogenous variation in this overlap. Our sample consists of firm-level data for roughly 20 million firms from over 40 countries over the period of two decades. We show that OLS estimates are strongly biased, yielding a positive correlation between taxation and output/efficiency. Accounting for the endogeneity via instrumenting yields robust negative estimates of the effects of taxation on firm output and efficiency. The results do not depend of firm characteristics, but are heterogeneous across countries: strong negative effects in some countries are accompanied by negligible or zero effects in others.