Patients with advanced cancer are influenced by the disease itself and by treatment side effects, both of which may have great impact on their lives. One of the most distressing symptoms is pain. However, pain in cancer patients can in most cases be relieved if the patient is able to communicate the nature and severity of the problem to the healthcare professionals through an effective assessment process. The main goal of this paper is to help form an understanding of central patient characteristics that should be taken into account when designing pain assessment tools for patients with advanced cancer. Traditionally, pain has been assessed by paper-based questionnaires and pain drawings. An iterative study was conducted based on repeated cycles of usability testing of a computerized pain body map for communicating pain by advanced cancer patients. Our aim was to provide a patient interface that most patients were able to interact with, collecting valuable, granular pain information with a minimum of strain on the patient. Through this process, we identified and solved design issues related to the sickest and frailest cancer patients. We further created a web-based solution for collecting individual pain drawings for evaluation by clinicians. The concept was appreciated by the patients, and the information provided was considered valuable by physicians. The main contribution of this paper is a list of suggestions to guide the design of an interactive tool for patients with advanced cancer.