The present research identifies the drivers of the growth of academic spin-offs (ASOs). Empirical studies have found few successful cases of ASOs from a firm-level perspective; many companies fail, and others tend to remain small without making a meaningful contribution to local economic development. Consequently, analysing the factors affecting spin-off growth is important to understand their potential benefits for society. A large portion of studies focus on external drivers of ASO growth, while we concentrate on the internal dimension, analysing the effect of the heterogeneity of entrepreneurial teams on the performance of ASOs from a longitudinal perspective. Our paper investigates how ASO entrepreneurial team heterogeneity, both horizontal (cognitive) and vertical (status), and changes in entrepreneurial team heterogeneity between the prefoundation and postfoundation stages affect ASO performance. The results show that vertical heterogeneity in the composition of the prefoundation team is a driver of ASO performance over a shorter time horizon (one year) and more so if the average seniority of academics is low. Horizontal heterogeneity plays a negative moderating role. However, longer-term performance is more influenced by horizontal heterogeneity, implying slower growth after the spin-off foundation. These findings should provide important insights for both managers and Italian policy-makers.