BACKGROUND: Little is known about how power outages affect health. We investigated mortality effects of the largest US blackout to date, 14-15 August 2003 in New York, NY. METHODS: We estimated mortality risk in New York, NY, using a generalized linear model with data from 1987-2005. We incorporated possible confounders, including weather and long-term and seasonal mortality trends. RESULTS: During the blackout, mortality increased for accidental deaths (122% [95% confidence interval Euro Surveillance (Bulletin Europeen Sur Les Maladies Transmissibles; European Communicable Disease Bulletin) 28%-287%]) and nonaccidental (ie, disease-related) deaths (25% [12%-41%]), resulting in approximately 90 excess deaths. Increased mortality was not from deaths being advanced by a few days; rather, mortality risk remained slightly elevated through August 2003. CONCLUSIONS: To our knowledge, this is the first analysis of power outages and nonaccidental mortality. Understanding the impact of power outages on human health is relevant, given that increased energy demand and climate change are likely to put added strain on power grids.