We use the lens of the resource-based view and horizontal agency cost theory to analyse the effect of the presence of different types of individual owners, i.e., owner-managers and non-manager individual shareholders, on the performance of high-tech entrepreneurial firms. Ownership enlargement may contribute to fill the resource gap faced by entrepreneurial firms and improve firm performance. However, whereas owner-managers engender low horizontal agency costs, non-manager individual shareholders generate high horizontal agency problems because of their limited managerial involvement. Our results on a sample of Italian high-tech entrepreneurial firms show that the number of owner-managers has a positive effect on firm performance, whereas the effect of the number of non-manager individual shareholders is negligible. This latter effect becomes more positive, even though still not statistically significant, when firms are highly leveraged confirming the disciplining role of bank debt.