The discipline of Human-Computer Interaction design has potential for significant benefit to the field of health informatics. This paper describes the design approach used to develop a web-based interface to help cancer patients manage their chemotherapy side effects. Previous versions of this intervention utilizing telephone technology had been efficacious, but limited. The paper discusses the design decisions made in order to leverage the potential benefits of the Internet in supporting patients while avoiding the potential pitfalls that the patients may encounter with a web-based approach.