Sweat equity is investment in a business that is financed by owner-workers being compensated at less than their market rate. Taking into account the hours spent building sweat equity while ignoring the output introduces an error in measured productivity and distorts the picture of what is happening in the economy. In this paper, we incorporate sweat equity in an otherwise standard business cycle model. We use the model to analyze productivity in the United States during the 1990s boom. We find that sweat investment was large during this period and critical for understanding the dramatic rise in hours and the modest growth in productivity.