We investigated the survival benefit bystander cardiopulmonary resuscitation (CPR) for out-of-hospital emergencies in a paramedic served area of metropolitan Los Angeles. Clinical information for all events occurring between January 1 and December 31, 1978 was obtained from paramedic report forms and hospital medical records. Bystander CPR was performed for 93 cases and, of these, 20 (22 per cent) survived to hospital discharge, as compared to 7 (5 per cent) of the 150 patients not receiving bystander CPR (p less than 0.001). Twelve (27 per cent) of the 45 patients in ventricular fibrillation (VF) who had bystander CPR survived, as compared to 4 (6 per cent) of 70 VF patients without bystander CPR (p less than 0.01). We conclude that bystander CPR, initiated prior to arrival of paramedics, produced a fourfold improvement in survival. Overall there was a 10 per cent survival rate at hospital discharge. Survival rates reported from Seattle may not necessarily be generalized to larger cities.