By: Editor:Key Takeaways * The 10 largest companies now make up 36% of the S&P 500 and 46% of the Nasdaq-100. * With this historically high concentration risk, buying traditional S&P 500 funds is no longer a truly balanced approach. * With this historically high concentration risk, buying traditional S&P 500 funds is no longer a truly balanced approach. The 10 largest companies in the Nasdaq-100, which holds the 100 largest non-financial companies listed on the Nasdaq exchange, account for 46% of its portfolio. For example, total market index funds like the Vanguard Total Stock Market Index Fund ETF (VTI) provide access to around 3,500 stocks.