In this study we investigate how the dispersion of family ownership among family members affects the performance of small-to-medium-size family firms. Based on agency theory and prior research on family firms, we develop arguments pointing to the existence of a U-shaped relationship between the degree of family ownership dispersion and firm performance. We also consider the involvement of family members in top management as a moderating factor of this relationship. The empirical analyses conducted on 494 small-to-medium size private family firms in Italy support our hypotheses and offer further evidence about curvilinear relationships between family ownership and family involvement in management, and performance. Overall, our study represents a theoretical synthesis and extension of the effects of family involvement on the performance of small-to-medium size private firms. It adds empirical evidence to this stream of research, offers new insights into the sources of heterogeneity among the population of family firms, and paves the way for future investigations on other organizational outcomes, especially firm growth, in family firms.