Using proprietary data on equity lending supply, loan fees and quantities we examine the link between institutional ownership structure and the market for equity lending and stock prices. We find that both total institutional ownership and ownership concentration - measured by the Herfindahl index, single largest holding, and number of investors - are important determinants of equity lending supply and short sale constraints. More concentrated ownership structures increase short sale constraints - including loan fees, recall risk and arbitrage risk - and forces arbitrageurs to decrease demand for equity borrowing and demand greater compensation for borrowing stock. The results show that the impact of institutional ownership structure in the equity lending market creates limits to arbitrage.