An H1N1 influenza outbreak was identified at a Veteran Benefits Administration (VBA)-sponsored conference in Baltimore, MD, in July 2009 in which affected VBA employees (both local and from out-of-town) sought healthcare at the VA Maryland Health Care System*Baltimore Medical Center. Using ESSENCE, daily counts of ICD-9 codes related to influenza diagnoses (as defined by VA ESSENCE influenzalike illness [ILI] syndrome group) were collected from the VHA Baltimore Medical Center from March 01, 2009, to September 12, 2009 . Data included case status (as defined by ICD-9 code and chart review), date and location of visit and patients' zip code of residence. We also accessed data from the VA Planning System and Support Group to determine whether the patients' residential ZIP code fell within the Baltimore VA Medical Center's catchment area. Using SAS, a p-chart (where the denominator was the daily number of patient ILI encounters) was run to determine days during which an aberrant proportion of patients from out-of-catchment zip codes were encountered.