The data collection of the 15th Italian Population Census was performed both through traditional on-field collection and via a web application that allowed citizens to fill in their questionnaire directly online. Data collected through the questionnaire was immediately sent to the central database at Istat upon saving. The online questionnaire was part of an integrated web system, entirely designed and developed by Istat, that included also the Census management system, used by enumerators, operators, coordinators, and Istat personnel to manage the various activities, and the online documentation systems available for operators. In this paper we focus on the design of the online questionnaire and present the innovative design solutions on which it was based, including a metadata-driven approach and a sophisticated mechanism to manage the flow of responses. Detailed statistics on the usage of the web application are also provided and commented.