In this study, we investigate empirically the relationship between ownership structure of Belgian listed firms, and their performance measured by Tobin's Q. We focus on the management and the largest shareholders equity ownership. We use first a cross-sectional estimation from 1991 to 1996. Second, we use panel data estimation to control whether the results found cross-sectionally are not due to unobserved firm heterogeneity. The use of panel data confirms the results obtained cross-sectionally for managerial ownership, that is, the relationship between the fraction of equities held by managers and Tobin's Q is negative. However, panel data results for the relationship between largest shareholders equities ownership and Tobin's Q become positive, while it is negative cross-sectionally. These results indicate that there is firm heterogeneity which is not captured in the cross-section estimation.