AIM: To describe the characteristics and outcome among patients suffering from in hospital cardiac arrest in relation to whether the arrest took place during office hours. PATIENTS: All patients suffering in hospital cardiac arrest in Sahlgrenska University hospital in Göteborg, Sweden between 1994 and 1999 in whom resuscitative efforts were attempted and for whom the time when the cardiopulmonary resuscitation (CPR) team was alerted. METHODS: Prospective recording of various factors at resuscitation including the time when the CPR team was alerted. Retrospective evaluation via medical records of patients previous history and final outcome. RESULTS: Among patients in whom the arrest took place during office hours (08:00-16:30 h) the overall survival rate was 49% as compared with 26% among the remaining patients (P<0.0001). The corresponding figures for patients found in ventricular fibrillation were 66 and 44% (P=0.0001), for patients found in asystole 33 and 22% (NS) and for patients found in pulseless electrical activity 14 and 3% (NS). When correcting for dissimilarities in previous history and factors at resuscitation the adjusted odds ratio for patients to be discharged alive who had the arrest during office hours was 2.07 (1.40-3.06) as compared with patients who had an arrest outside office hours. CONCLUSION: Among patients suffering from in hospital cardiac arrest and in whom CPR was attempted those who had the arrest during office hours had a survival rate being more than twice that of patients who had the arrest during other times of the day and night. These results indicate that the preparedness for optimal treatment of in hospital cardiac arrest is of ultimate importance for the final outcome and that an increased preparedness during evenings and nights might increase survival among patients suffering from in hospital cardiac arrest.