In countries where the Internet access is high, a web-based questionnaire could save time and money compared to printed questionnaires, mainly by eliminating the two steps of transferring answers from printed to a digital data set and manually completing missing and impossible answers. However, many of the features wanted for conducting large epidemiological studies are not available in many web survey systems. Here we describe design issues the investigator needs to be aware of when using web-based questionnaires in epidemiological research.