Little is known about how young high-tech ventures create openness in their knowledge networks. This paper explores the influence of antecedent resources on openness in knowledge networks, seen as diversity in knowledge partners, and explores the impact of openness on growth. The results from 105 university spin-off firms suggest that three antecedents positively influence openness, namely, founders' prestart experience, education and innovation experience, and one negatively, namely, size of the founding team. Regarding non-linearity, there are signs of cubic influences, potentially in line with passing critical junctures. In addition, external factors tend to have no influence on openness, except for region of location. Further, shaping the right amount of openness and benefitting from it seem a struggle, as an increasing openness tends to influence growth with decreasing returns.