16% of tax payers are thought to cheat on their taxes.

The IRS estimates that only about 84% of tax payers are being honest when they file their tax returns.
Source:
http://news.yahoo.com/s/livescience/20100414/sc_livescience/profileofataxcheat;_ylt=ApVmZikdg7UMc5jia1Al.Qes0NUE;_ylu=X3oDMTN2bThnbWYxBGFzc2V0A2xpdmVzY2llbmNlLzIwMTAwNDE0L3Byb2ZpbGVvZmF0YXhjaGVhdARjY29kZQNtb3N0cG9wdWxhcgRjcG9zAzcEcG9zAzQEcHQDaG9tZV9jb2tlBHNlYwN5bl9oZWFkbGluZV9saXN0BHNsawN0YXhkYXljb21ldGg-