In May 2009, we investigated a hospital outbreak of pandemic H1N1 (pH1N1) infection among healthcare personnel (HCP). Thirteen (65%) of 20 HCP with pH1N1 infection had healthcare-associated cases, which were primarily attributed to transmission among HCP. Eleven (55%) of HCP with pH1N1 infection worked for 1 day or more after the onset of illness. Personnel working with mild illness may have contributed to transmission among HCP.