The assumption that research findings provide the basis for spin-off projects at universities has been found up to now in literature and the practice. Supported by the theory of knowledge, the empirical study presented here shows that this idea is too limited. Only 45 % of spin-offs use codified research findings from the university, while 55 % use tacit knowledge that was acquired at the university. These spin-offs use knowledge beyond research findings, starting companies in the shadow of publications by academic institutions and drawing from the realm of tacit knowledge at universities. Tacit start-up knowledge is present in all scientific disciplines of universities; even the exploitation- and patent-oriented engineering sciences account for almost half of the start-ups. Start-ups based on tacit knowledge lead to both technology-oriented and service companies. They also do not differ from codified knowledge-based start-ups in the number of jobs that they create. The discovery of the tacit knowledge spin-offs as a phenomenon has an entire series of implications for the practice and research. The tacit start-up potential was not considered previously in the university promotion instruments and start-up consultancies. Furthermore, we can assume that tacit knowledge-based start-ups are only an initial indication of the innovation potential within the tacit realm of knowledge for universities and research institutes.