This paper focuses on contributions of users in early phases of radical innovation projects. In a multiple case study analysis in the field of medical equipment technology, we identify characteristics of users who contribute substantially to the development of radical innovations by being their inventors and (co)-developers. These innovative users have high motivation to seek new solutions, possess a diverse set of competencies, and are embedded in a supportive environment. We furthermore observe that they play an entrepreneurial role as they establish and organize the required innovation networks. These innovation networks are needed to transform the users' radically new concepts into first physical prototypes and marketable products. The study highlights how manufacturing firms can benefit from innovative and entrepreneurial users in the early phases of radical innovation projects.