Using U.S. data from 1960 to 2007 this empirical note re-examines the semi-endogenous and Schumpeterian R&D growth models presented by Ha and Howitt (2007) and Madsen (2008). The empirical results support the Schumpeterian R&D growth model. Specifically, in the long-run increases in R&D expenditures are necessary to counteract lower R&D productiv-ity due to the presence of product proliferation. Furthermore, the study provides a frame-work for further investigation of R&D growth models at the regional level.