The purpose of this study was to evaluate a colorimetric end tidal COP (ETCO2) detector (EASY CAP) as a monitor during prehospital cardiopulmonary resuscitation (CPR) without tracheal intubation. This detector was used for 121 patients during CPR with a laryngeal mask airway or face mask by authorized emergency lifesaving technicians. At 7 to 15 minutes after the initiation of CPR, ETCO was <0.5% in 30 cases (group A), 0.5% to 2.0% in 46 cases (group B) and >2.0% in 45 cases (group C). The rate of return of spontaneous circulation was 17% in group A, 24% in group El, and 48% in group C (groups A v C, P < .01). There was a significant difference in the rate of hospital admission between groups A and C. the ETCO2 value may be useful for monitoring during prehospital CPR with a laryngeal mask airway or face mask. Copyright (C) 1999 by W.B. Saunders Company.