In this paper we explore a matched employer-employee data set to investigate the presence of gender wage discrimination in the Belgian private economy labour market. We identify and measure gender wage discrimination from firm-level data using a labour index decomposition pioneered by Hellerstein and Neumark (1995), which allows us to compare direct estimates of a gender productivity differential with those of a gender labour costs differential. We take advantage of the panel structure of the data set and identify gender wage discrimination from within-firm variation. Moreover, inspired by recent developments in the production function estimation literature, we address the problem of endogeneity in input choice using a structural production function estimator (Levinsohn and Petrin, 2003). Our results suggest that there is no gender wage discrimination inside private firms located in Belgium.