Aim To describe how unannounced in situ simulation (ISS) was perceived by healthcare professionals before and after its implementation, and to describe the organisational impact of ISS. Study design Ten unannounced ISS involving all staff were scheduled March-August 2007. Questionnaire surveys on staff perceptions were conducted before (2003-2006) and after (2007-2008) implementation of unannounced ISS. Information from the debriefing sessions following each ISS constituted a proxy measure of the organisational impact of the ISS. Results Five out of ten of the unannounced ISS scheduled were conducted. Twenty-three members of the staff at work on a scheduled day for ISS were randomly selected to participate. Questionnaires before implementation revealed that 137/196 (70%) of staff members agreed or strongly agreed that ISS was a good idea and 52/199 (26%) thought it likely to be stressful and unpleasant. Questionnaires completed after implementation showed significantly more staff members, 135/153 (89%), thought ISS was a good idea. A significantly higher amount of staff members 50/153 (33%) found it to be stressful and unpleasant, and among midwives, 15/59 (25%) were anxious about ISS, whereas none of the obstetricians reported this. Information obtained through debriefing sessions generated learning points. Conclusions The number of staff members with a positive perception of multiprofessional unannounced ISS increased after implementation; however, one-third considered ISS to be stressful and unpleasant and midwives more frequently so. The specific perception of ISS by each healthcare profession should be taken into account when planning ISS. The information from the debriefing sessions showed that implementation of ISS had an impact as it provided information required for organisational changes.