We explore the impact on innovation that professional end-users of a product have as inventors, executives, and board members in a young firm. In contrast to prior literature, which has emphasized technology roles, we put the spotlight on the executive and governance roles that many professional users take in young firms. Using an extensive custom-collected dataset of 231 surgical instrument ventures over a 25-year period, we find that professional users strengthen innovation in some roles but block it in others. Physician-users are related with the increase in a firm's innovation when they take a technology role as inventors, and particularly when they take a governance role on the young firm's board. However, despite their frequent involvement in executive roles, physician-executives are less likely to be helpful, and especially likely to block innovation as chief executives. Our results emphasize professional users as a critical external dependency for a young firm's innovation, but show that a mismatch with a particular organizational role may have unanticipated negative effects on innovation. In their roles, users can be helpful by expanding the variety of solutions to the firm's innovation problems, but are significantly less helpful in just improving their selection. Our findings have implications for research on evolutionary perspective on user innovation, organizational roles in young firms, and entrepreneurial policy.