A key organizational barrier related to the implementation of open innovation strategies refers to the unwillingness of employees to undertake extra-organizational knowledge transactions. Negative attitudes against the utilization of external knowledge (i.e. the Not-invented-here (NIH) syndrome), as well as against the external commercialization of knowledge assets, for example, via licensing (i.e. the Not-sold-here (NSH) syndrome), may create resistance to these activities and, consequently, a misalignment between the intentions of top management and the attitudes of involved employees (Katz and Allen, 1982; Lichtenthaler et al., 2010). In this paper, we examine the extent to which these attitudes impact the actual adoption of both the inbound and the outbound approaches to open innovation. We posit that these attitudes have a negative influence, since they create unfavourable perceptions of the value of outside competencies and know-how, supporting only internal development and application of ideas and technologies. We test two hypotheses concerning the consequences of the NIH- and NSH-syndromes with cross-sectional survey data from 355 Danish firms. The population consists of firms in the manufacturing industries (NACE codes 10-37) with 5-499 employees. Our findings help explain the extent to which companies are able to benefit from inflows and outflows of knowledge.