Background and Objective.Assessing the impact of 2009 influenza A (H1N1) on healthcare workers (HCWs) is important for pandemic planning.Methods.We retrospectively analyzed employee health records of HCWs at a tertiary care center in New York City with influenza-like illnesses (ILI) and confirmed influenza from March 31, 2009, to February 28, 2010. We evaluated HCWs' clinical presentations during the first and second wave of the pandemic, staff absenteeism, exposures among HCWs, and association between high-risk occupational exposures to respiratory secretions and infection.Results.During the pandemic, 40% (141/352) of HCWs with ILI tested positive for influenza, representing a 1% attack rate among our 13,066 employees. HCWs with influenza were more likely to have fever, cough, and tachycardia. When compared with the second wave, cases in the first wave were sicker and at higher risk of exposure to patients' respiratory secretions (P = .049). HCWs with ILI- with and without confirmed influenza-missed on average 4.7 and 2.7 work days, respectively (P = .001). Among HCWs asked about working while ill, 65% (153/235) reported they did so (mean, 2 days).Conclusions.HCWs in the first wave had more severe ILI than those in the second wave and were more likely to be exposed to patients' respiratory secretions. HCWs with ILI often worked while ill. Timely strategies to educate and support HCWs were critical to managing this population during the pandemic.