BACKGROUND: Real-world evidence of out-of-hospital ventricular fibrillation (VF), especially regarding intra-day change, remains unclear. We aimed to investigate that age- and gender-dependent difference of intra-day change of VF occurrence. METHOD: We enrolled 71,692 patients (males: 56,419 [78.7%], females: 15,273 [21.3%]) in whom cardiac VF had been documented from the 2005-2015 All-Japan Utstein Registry data. Subjects were divided into four groups: group-I (<18 years old), group-II (18-39), group-III (40-69), and group-IV (≥70). Among four groups in each of male and female, we compared the intra-day change of VF occurrence, and evaluated the risk factors of the unfavorable neurologic outcomes at 1 month after VF. RESULTS: Regardless of age, the incidence of VF was significantly greater in male than in female subjects. In male subjects, VF in group-I, III and IV occurred higher at daytime, however, group-II had no intra-day difference because group-II had a higher VF events at midnight~ early morning compared with other aged groups (Poisson regression analysis, p = .03). While in female, each group showed similar intra-day pattern of VF occurrence. Logistic regression analysis revealed that some of the clinical parameters such as time periods from call receipt to first shock and the presence of bystander cardiopulmonary resuscitation were important for risk of 30-day neurologically unfavorable outcomes. CONCLUSIONS: The intra-day change of VF occurrence was age-dependently different in males but not in females, suggesting age- and gender-dependent differences in underlying cardiac diseases. These might affect the significant difference in unfavorable neurologic outcome.