No previous study has been able to examine the association by taking account of risk factors for dementia before and after the disaster. We prospectively examined whether experiences of a disaster were associated with cognitive decline in the aftermath of the 2011 Great East Japan Earthquake and Tsunami. The baseline for our natural experiment was established in a survey of older community-dwelling adults who lived 80 km west of the epicenter 7 mo before the earthquake and tsunami. Approximately 2.5 y after the disaster, the follow-up survey gathered information about personal experiences of disaster as well as incidence of dementia from 3,594 survivors (82.1% follow-up rate). Our primary outcome was dementia diagnosis ascertained by in-home assessment during the follow-up period. Among our analytic sample (n = 3,566), 38.0% reported losing relatives or friends in the disaster, and 58.9% reported property damage. Fixed-effects regression indicated that major housing damage and home destroyed were associated with cognitive decline: regression coefficient for levels of dementia symptoms = 0.12, 95% confidence interval (CI): 0.01 to 0.23 and coefficient = 0.29, 95% CI: 0.17 to 0.40, respectively. The effect size of destroyed home is comparable to the impact of incident stroke (coefficient = 0.24, 95% CI: 0.11 to 0.36). The association between housing damage and cognitive decline remained statistically significant in the instrumental variable analysis. Housing damage appears to be an important risk factor for cognitive decline among older survivors in natural disasters.instrumental variable analysis U p to two-thirds of the affected populations in the 2011 Great East Japan Earthquake and Tsunami were older residents who were 60 y old or older (1). Recovery after major disaster poses a unique set of challenges for the elderly population, including disruption of medical care for preexisting conditions, preexisting functional limitations that impede recovery, and social isolation in the aftermath of housing loss and resettlement. A particular concern for older survivors is the potential risks of cognitive decline. In the 2011 earthquake and tsunami, an estimated 340,000 residents were displaced as a result of widespread destruction to residential properties. In turn, as a direct consequence of residential dislocation and resettlement in unfamiliar surroundings, many seniors experienced disorientation that could hasten cognitive decline (2). Psychological trauma, including posttraumatic stress disorder (PTSD) symptoms (3) and the onset of depression (4), may have additionally contributed to this risk.Two years after the 2011 Great East Japan Earthquake and Tsunami, a cross-sectional study found that 36.0% of seniors who moved to temporary housing in Kesen-numa city were suffering from dementia symptoms (5). Another cross-sectional study of seniors affected by the disaster reported that 47.9% showed signs of mild cognitive impairment, and an additional 16.0% of respondents were diagnosed as having dementia (6). However, ...