Task-technology fit theory considers how technology may best be deployed to support individuals and facilitate the completion of tasks. This study separates the fit construct into the two realms of task-technology fit and technology-individual fit and integrates organization readiness with the objective of investigating the effectiveness of mobile nursing information systems in terms of helping nursing staff to accomplish daily clinical tasks. Study participants were clinical professionals with system usage experience who work at one medical center. Results indicated that technology-individual fit is the factor that most strongly influences usage, followed respectively by task-technology fit and organization readiness. Therefore, strategies designed to implement mobile nursing information systems should focus greater effort on fitting the system to system users by making these systems easy to learn and use, and training easy to complete. System functions should not only facilitate accomplishment of daily clinical tasks such as quickly obtaining information and accurate data but also be portable and provide a user-friendly, easy-to-operate interface. Organizational readiness, the commitment and support of top management, and nursing staff willingness to learn and use the new system are also important factors that influence system usage.