We introduce a new measure of the extent of federal regulation, the number of pages in the Code of Federal Regulations, and use it to investigate the relationship between federal regulation and macroeconomic performance in the U.S. We find that regulation has statistically and economically significant effects on aggregate output and the factors that produce it - total factor productivity, physical capital, and labor. In particular, regulation reduces the annual trend rate of growth in output by seven- to nine-tenths of a percentage point, implying a large annual cost (currently $5.4 trillion) in foregone annual output. Regulation also has complicated dynamic effects on all variables. The effects differ across factors of production, implying both dynamic and ultimate effects on resource allocation.