The not‐invented‐here (NIH) syndrome refers to a negative attitude of employees against externally developed knowledge. We show that the sources of external knowledge and the success of the company that acquires knowledge externally are factors that impact the occurrence of the NIH syndrome. In line with social identity theory, we hypothesize that internal resistance is most likely to occur if knowledge is acquired from similar organizations. This hypothesis is supported by our empirical finding that internal resistance against external knowledge is more likely to occur when knowledge is acquired from competitors rather than from suppliers, customers, or universities. Further, we show that the NIH syndrome is more likely to arise within successful companies that acquire knowledge from competitors. This is in line with our hypothesis that firm success increases the extent to which employees identify themselves with their company, resulting in stronger in‐group favoritism and a superior tendency to reject externally generated knowledge.