We examine the role of attendance in home-field advantage for Major League Baseball, using a dataset of all MLB games played from 1996 to 2005. Using two-stage least squares, we find that attendance has a significant effect on the home-field advantage. Our results indicate that a one standard deviation increase in attendance results in a 4% increase in the likelihood of a home team win. We also find that if attendance as a percent of stadium capacity were to increase by 48%, we would expect the home team's run differential to increase by one run. We show that the additional home-field advantage is driven by increased home team performance.