Open-chest cardiopulmonary resuscitation (CPR) is a promising method for non-traumatic cardiac arrest. In this preliminary study, we investigated the optimum timing of thoracotomy which brings high rate of return of spontaneous circulation (ROSC) and keeps the incidence of unnecessary thoracotomy minimal. Ninety-five adult patients with non-traumatic out-of-hospital cardiac arrest were analyzed. Of those, 26 patients were treated by the open-chest CPR in a prospective consecutive fashion. In this group, the ROSC rate was investigated in connection with the interval from hospital arrival, or ambulance call, to thoracotomy. Another 69 patients were treated by standard CPR. In this uncontrolled group, the interval from arrival at hospital to ROSC was investigated to define the 'natural hospital course' by the conventional treatment. Patient characteristics in the open-chest CPR group and the standard CPR group were similar. In the open-chest CPR group, 15 patients obtained ROSC. There was a tendency that the ROSC rate was highest in the patients with thoracotomy within 5 min of hospital arrival and declined as the timing of thoracotomy was delayed. Similar tendency was noted when the timing of thoracotomy was counted from the ambulance call. In the standard CPR group, only two patients obtained ROSC during the initial 5 min of hospital course. These results suggest that thoracotomy within 5 min of hospital arrival brings the highest ROSC rate while keeps the incidence of unnecessary thoracotomy acceptable.