We examined the public health effects of the Northeast blackout of August 2003 and the emergency response to the blackout by the New York City Department of Health and Mental Hygiene (DOHMH). We reviewed departmental documents from the DOHMH Emergency Operations Center and surveyed DOHMH employees to identify deficiencies in the response and elicit suggestions for improvement. DOHMH deployed its all-hazards, scalable public health Incident Management System to respond to several impacts: (1) failure of multiple hospital emergency generators; (2) patients dependent on electrically powered equipment; (3) loss of electronic data input to the DOHMH syndromic surveillance system from hospital emergency departments; (4) potential for vaccine spoilage due to loss of refrigeration; (5) beach contamination with untreated sewage; (6) heat-related health effects and increase of foodborne disease; and (7) potential for an increased rodent population as a result of increased amounts of discarded perishables. Areas identified for improvement included communications during the event, DOHMH dependence on an external source of electricity, facility management during the response, and lack of readily available and appropriate emergency supplies.