Risk assessments of metals in sediments are often based on sediment-quality guidelines (SQGs) and do not take into account the chemistry of the overlying water. To determine the effects of water chemistry on the toxicity of metals in sediments, both water and sediment were collected from five metal-contaminated lakes with widely differing water chemistry near Canadian smelters. Metal bioaccumulation by Hyalella azteca was measured in laboratory exposures with each sediment and with overlying water from several different sources. The effect of water chemistry on Ni bioaccumulation from sediment was minimal. However, the effect was substantial for Cd because the effect of water chemistry on sediment-water partitioning was opposite to, and augmented, the effect of water chemistry on the Hyalella/water accumulation ratio. The effect of overlying water must be considered when conducting risk assessments for some metals in sediment (e.g., Cd). Examples are provided of equations that can be used to adjust cause-and-effect-based SQGs for water chemistry (e.g., using calcium concentrations or pH).