Skilled workers' mobility is considered to be one of the most influential channels of knowledge transmission. This has increased the interest of researchers and policy makers because of its implications for innovation diffusion and, consequently, economic welfare. However, little is still known about the determinants of this phenomenon. This paper explores the mobility patterns for a group of Italian inventors in the pharmaceutical sector. It addresses methodological issues related to measures of inventors' mobility through patent statistics and examines the determinants of their mobility choices. The empirical results indicate that career paths of inventors are rarely reflected in their patenting activity and that using patent statistics frequently underestimates the intensity of the mobility phenomenon. The results also show a positive association between productivity and mobility. In particular, the econometric analysis points out that inventor's personal characteristics, inventive productivity, and geographical location matter for mobility choices.