Abstract Background Mortality rates in Osaka for cardiac arrest after witnessed ventricular tachycardia (VT) or ventricular fibrillation (VF) have decreased dramatically. We sought to estimate the contribution of changes in out-of-hospital care to this decrease. Methods We applied a previously validated statistical model, IMPACT, to data obtained from the Utstein Osaka Project, which registers all cardiopulmonary arrests in Osaka. The outcome was death within the first month after the arrest. Sensitivity analysis was conducted by simulating an increase in the use of public access defibrillators (PADs). Results From 1999 through 2008, age- and sex-adjusted standardized 1-month mortality fell from 88.6% to 57.1%. There were 105 fewer deaths than expected in 2008 (295 deaths). The IMPACT model explained 62.5% of the decrease (67 deaths) in the 1-month mortality. The main contributors to the decrease in mortality were an increase in the use of biphasic waveform defibrillators, and a shortened time to first shock. These were partly offset by an increase in the administration of epinephrine by emergency medical services personnel. According to the simulation, an increase in PAD use from 1.9% to 34.4% would reduce mortality from the observed 57.1% to 49.5%. Conclusions Modeling suggests that improvement in out-of-hospital care accounted for approximately 60% of the decline in deaths following witnessed VT or VF arrests in Osaka between 1999 and 2008. Increased usage of PADs could further improve these outcomes.