Abstract: We examine adverse selection costs around NYSE decimalization. Further, we analyze the relation between adverse selection costs and trade size. We find a significant increase in the percentage adverse selection cost and a reduction in dollar adverse selection cost (percentage adverse selection multiplied by the spread) following complete decimalization on the NYSE. On estimating the adverse selection components by trade size classes, we find a decline in dollar adverse selection costs in trades of all sizes, with the strongest evidence coming from medium size trades, followed by small and large size trades. One implication of our findings is that there appears to be less stealth trading following complete decimalization and less institutional trading overall.