Clinical use of computer systems has been hampered by their poorly designed user interfaces. The objective of this study was to design a user interface for a pediatric oncologists' computerized patient record with great consideration of their working behavior and of human computer interfacing principles so as to contribute to oncologists' efficiency and satisfaction in interaction with the system. The think aloud method was used in combination with video recording to get a deep understanding of the way in which four pediatric oncologists searched through the paper-based patient record in preparing a patient visit. Protocol and video analyses was used to develop a cognitive task model reflecting pediatric oncologists' task behavior. This model was input for a prototype user interface, which was subsequently evaluated by eight other pediatric oncologists. The resulting computerized medical record system proved to meet pediatric oncologists' information needs and task behavior patterns. The design of the user interface minimized pediatric oncologists' work load and was highly efficient in supporting the pediatric oncologists in preparing their patient visits. The pediatric oncologists were very much satisfied with the computer system. It is argued that early involvement of cognitive engineering methods in the system design process may be of great help in designing systems that fully support health care professionals' work practices. The think aloud method, if applied under prescribed conditions, is a valuable information source of human task-behavior and as such a useful technique for requirements analysis in designing clinical computer systems.