Warren Buffet has said the same thing - he has no problem with releasing returns that are under audit.

While I don't like the "what's he hiding" argument, fact is his returns could show a number of things that he doesn't want the public to know.  The most ob ious:  He's not as wealthy as he'd like to make out (fact is, he sued one author who did a little digging and found out that Trump's net worth was nowhere near what he claimed).  He's got ties with some unsavory people (Russian or otherwise).  He really has been cheating on his taxes all these years.