Mike O'Sullivan at Corporate Law Blog has a good short commentary on two recent GAO reports about the accounting industry.

it found that the Big-4 accounting firms audit 78% of U.S. public companies. This 78% includes virtually every firm of any consequence -- the Big-4's public company audit clients account for an amazing 99% of all sales reported by U.S. public companies. The gulf separating the Big 4 firms from the others is wide -- the smallest Big 4 firm (KPMG) had revenues of $3.2 billion, 8 times the revenues of the next biggest firm (Grant Thornton with $400 million).