AIM: To describe rhythm changes during the initial phase of resuscitation from ventricular fibrillation in relation to the interval between collapse and defibrillation, to survival and to bystander-initiated cardiopulmonary resuscitation (CPR). PATIENTS: All patients who suffered out-of-hospital cardiac arrest between 1980 and 1992, who were reached by the emergency medical service system (EMS), in whom resuscitation attempts were initiated and who were found in ventricular fibrillation. RESULTS: In all, 1216 patients were included in the study. Among patients who converted to a pulse-generating rhythm after the first defibrillation (n = 119) were 56% discharged from hospital as compared with 6% among patients who converted to asystole. The corresponding figures after the third defibrillation were 49% and 2%, respectively, and after the fifth defibrillation 28% and 7%, respectively. Among patients in whom the first defibrillation took place less than 5 min after collapse, 28% directly converted to a pulse-generating rhythm as compared with 3% when the first defibrillation took place 12 min or more after collapse. CONCLUSION: Among patients who suffer out-of-hospital cardiac arrest and are found in ventricular fibrillation, there is a strong relationship between survival and initial rhythm changes after defibrillation. These rhythm changes are directly related to the interval between collapse and the first defibrillation.