After identification of pandemic 2009 influenza (pH1N1) in the United States, the Centers for Disease Control and Prevention (CDC) worked with state and local health officials to characterize infections among healthcare personnel (HCP). Detailed information, including likely routes of exposure, was reported for 70 HCP from 22 states. Thirty-five cases (50%) were classified as being infected in healthcare settings, 18 cases (26%) were considered to have been infected in community settings, and no definitive source was identified for 17 cases (24%). Of the 23 HCP infected by ill patients, only 20% reported using an N95 respirator or surgical mask during all encounters and more than half worked in outpatient clinics. In addition to community transmission, likely patient-to-HCP and HCP-to-HCP transmission were identified in healthcare settings, highlighting the need for comprehensive infection control strategies including administration of influenza vaccine, appropriate management of ill HCP, and adherence to infection control precautions.