This paper presents empirical estimates of human-capital augmented growth equations for a panel of 21 OECD countries over the period 1971-98. It uses an improved dataset on human capital and a novel econometric technique that reconciles growth model assumptions with the needs of panel data regressions. Unlike several previous studies, our results point to a positive and significant impact of human capital accumulation to output per capita growth. The estimated long-run effect on output of one additional year of education (about 6 per cent) is also consistent with microeconomic evidence on the private returns to schooling. We also found a significant growth effect from the accumulation of physical capital and a speed of convergence to the steady state of around 15 per cent per year. Taken together these results are not consistent with the human capital augmented version of the Solow model, but rather they support an endogenous growth model à la Uzawa-Lucas, with constant returns to ...