~~Using a longitudinal data base, we find that there has been a substantial narrowing of the spread between the price-cost margins of concentrated and unconcentrated industries. Including measures of macroeconomic fluctuations into standard price-cost margin regressions, we find significant differences in cyclical behavior across market structure. We also explore a possible explanation, unionization, for differences in cyclical behavior. Finally, we investigate the degree to which our results are affected by the potential simultaneity of concentration, advertising, and the price-cost margin.