From 2002 statistics from the IRS:
The IRS data divide taxpayers into percentiles according to their adjusted gross incomes. Following is the share of aggregate income taxes paid by each group:
Income Group ---- Tax Share
Top 1 percent ---- 33.7 percent
Top 5 percent ---- 53.8 percent
Top 10 percent ---- 65.7 percent
Top 25 percent ---- 83.9 percent
Top 50 percent ---- 96.5 percent
The data also reveal that despite the Bush tax cuts, the income tax is still highly progressive -- taking more from each group as their incomes rise. The following percentages measure the taxes paid by each group divided by their income. Economists call this the average or effective tax rate.
Income Group ---- Tax Rate
Top 1 percent ---- 27.25 percent
Top 5 percent ---- 22.95 percent
Top 10 percent ---- 20.51 percent
Top 25 percent ---- 16.99 percent
Top 50 percent ---- 14.66 percent
Bottom 50 percent ---- 3.21 percent