The article examines how concentration affects profitability in US banks from 1994-2005. A new concentration index is introduced to measure the depth of concentration. The study supports the idea that higher concentration leads to higher profitability in the banking sector. Even excluding the largest banks, concentration still boosts profitability for large and small banks. Concentration also impacts interest rates, revenue, and costs in banks. Overall, the findings suggest that bank concentration can have both positive and negative effects on savings, investment, and growth.