We examine whether there are increasing returns to locating in clusters-a fundamental premise of agglomeration theory. This premise is worth exploring because a) it has infrequently been studied; b) recent evidence suggests that diseconomies of agglomeration change over time and may alter the payoffs to firms located in clusters; and c) recent evidence suggests that there may be negative returns to agglomeration. We examine firm payoffs to clustering in the biotechnology industry, and consider five different measures of performance. Our results lend support to the view that there are increasing returns to cluster size, but also suggest that diseconomies of agglomeration play an increasingly important role as clusters evolve. We believe these results help in reconciling previous findings. (c) 2005 Published by Elsevier Inc.