We investigate the relation between Tobin's Q and the structure of equity ownership for a sample of 1,173 firms for 1976 and 1,093 firms for 1986. We find a significant curvilinear relation between Q and the fraction of common stock owned by corporate insiders. The curve slopes upward until insider ownership reaches approximately 40% to 50% and then slopes slightly downward. We also find a significant positive relation between Q and the fraction of shares owned by institutional investors. The results are consistent with the hypothesis that corporate value is a function of the structure of equity ownership.