Although an unambiguous and consistent representation is the foundation of data reuse, a locally developed documentation system such as nursing flowsheets often fails to meet the requirement. This article presents the domain modeling process of the ICU nursing flowsheet to clarify the meaning that its contents represent and the lessons learned during the activity. This study has been done as a first step toward reusing the data documented in a computerized nursing flowsheet for an algorithmic decision making. Following the ontology development processes proposed by other researchers, a conceptual model was developed using Protégé. Then, the existing information model was refined by fully specifying the embedded information structures and by establishing linkages to the conceptual model at the finest-grained concept level. Domain knowledge that the experienced nurses provided was critical to correctly interpret the meaning of the flowsheet contents as well as to verify the newly developed models. This study reassured the importance of the roles of a nurse informaticist to develop a computerized nursing documentation system that accurately represents the information needs in nursing practice.