OBJECTIVE: Japan has one of the most rapidly aging societies worldwide. This study aimed to assess the long-term nationwide trend of out-of-hospital cardiac arrest (OHCA) cases among elderly patients in Japan. METHODS: This prospective, nationwide observational study in Japan included elderly patients aged ≥65 years who experienced OHCA from January 1, 2005 to December 31, 2015. The patients were classified into three groups: young-old, aged 65-74 years; old-old, aged 75-84 years; and oldest-old, aged ≥85 years. The primary outcome of this study was one-month survival with a favorable neurological outcome, which was defined as a cerebral performance category scale score of 1 or 2. RESULTS: A total of 877,009 patients were included in our analysis. The number of elderly patients with OHCA increased from 65,968 in 2005 to 87,339 in 2015, and each age category showed a significantly increasing trend (p value <0.001 for each trend). The proportions of favorable neurological outcomes also increased from 1.2% in 2005 to 2.8% in 2015 in the young-old group; from 0.6% in 2005 to 1.1% in 2015 in the old-old group; and from 0.2% in 2005 to 0.5% in 2014 in the oldest-old group. Furthermore, this improving trend was notable for those with a shockable first documented rhythm. CONCLUSIONS: Based on this long-term nationwide observational study in Japan, the number of elderly patients with OHCA increased annually, and a significant improvement in the patients' neurological outcomes was noted regardless of age category, particularly among those with an initially shockable rhythm.