Analysis of interactivity in Web sites is an important extension of a long tradition of analyzing content of media messages. But both interactivity and online analysis of content and features offer unique challenges to researchers. This study develops and tests a tool for measuring interactivity in the context of health-related Web sites. The tool was flexible enough to distinguish among multiple types of interactivity and powerful enough to show differences in interactivity based on domain type. Thus, it should have a relatively long life as a multifaceted tool for the tough job of measuring interactivity online.