This work assesses empirically the various explanations for why productivity growth is procyclical. Some theories of business cycles state that productivity movements reflect changes in technology and are the driving force of economic fluctuations. This study investigates whether price-cost margins, externalities, or slow adjustment of the labor market can account for the behavior of productivity growth over the cycle. The application of standard econometric techniques to two different measures of productivity growth in American manufacturing industries, one based on quantities and one based on prices, shows that there is some labor hoarding, which is compatible with either margins or externalities.