The objective of this study was to analyze the impact of medication ordering and administration functions of CPOE on doctor-nurse communications and cooperation. We performed an extensive analysis of the work situations in several departments of three different hospitals. One of the hospitals is still using paper-based orders, the second one is currently implementing a patient care information system (PCIS), but the analysis was carried out with paper-based orders. The third hospital has a PCIS installed with available medication ordering functions. We used standard methods from cognitive psychology to analyze physicians, and nurses' activity, communications and cooperation. This approach was combined with a usability analysis of both work systems, paper and computer-based. The paper-based situation is characterized by a synchronous cooperation with a distributed decision-making where physicians and nurses rely mostly on verbal communications to coordinate their actions; paper order sheets are weakly structured and poorly support the documentation task. In the computer situation, physicians and nurses work in an asynchronous mode, and leave to the system the coordination of their actions. Orders are exhaustively documented but some data may be misinterpreted. Some of these problems are due to usability flaws of the Human Computer Interface. We conclude with recommendations for usability improvement of CPOE systems, combined with recommendations for the organization of doctor-nurse communication when implementing such systems.