Recent trends in empirical macroeconomics research—embedding long-run relationships in models via cointegration, modeling the correlation between seasonal cycles and business cycles, building endogenous growth models, and the interest of policymakers in inflation targeting—have increased the importance of long-time series of macroeconomic data. Among the more important of such data are quantitative measures of monetary policy, such as the adjusted monetary base. Previously published data for the Federal Reserve Bank of St. Louis adjusted monetary base begin in 1935 (seasonally unadjusted), and in 1950 (seasonally adjusted). The authors develop a consistent time series for the adjusted monetary base that begins in 1918, shortly after the founding of the Federal Reserve System.