The present paper deals with an experience of blackout of the hospital information system at our Medical Centre, trying to evaluate the impact on internal users (physicians, nurses, clerks) and patients population. Limited inconveniences have occurred to out-patients in terms of delay in collecting medical reports after diagnostic test execution. As regards direct users, impact was evaluated through a structured interview. Administrative personnel, that have been using computer-based system for at least ten years, have not declared particular inconveniences, accepting the overtime or the extra-work as simply unavoidable. On the contrary, health-care personnel reported a heavy negative impact of the system failure on their activity. After a great effort to achieve the system acceptance and direct physicians usage, the blackout of the system has pointed out that the situation has changed since a few years ago: now the HIS, and particularly its clinical core, is considered mission critical.