Research and public policy on academic entrepreneurship in the United States is based on the assumption that the entrepreneurial activity of U.S. academics is accurately represented by efforts of faculty to commercialize inventions that they have disclosed to university administrators and that, in most cases, have been patented. In this paper, we analyze a sample of 11,572 university professors, representative of the entire population of academics affiliated to Carnegie I and II United States universities, and we find that a large part of the academic entrepreneurship activities occur outside the IP-system. In particular, a large proportion of businesses started by academics (about 2/3) are not based on disclosed and patented inventions. Moreover, we show that individual characteristics, departmental and organizational affiliation, as well as working time allocation, of academics starting business outside the IP-system are systematically different from those who have started businesses on disclosed and patented inventions. These findings have implications for research on academic entrepreneurship, as well as technology transfer policies.