Powassan virus (POWV) is a tickborne Flavivirus that causes a rare but potentially life-threatening illness. The first reported case of POWV infection in a Wisconsin resident occurred in 2003. Enhanced surveillance and testing detected 2 additional cases.Patient specimens with a positive or equivocal immunoglobulin M (IgM) antibody to an arbovirus were sent from commercial laboratories to the Wisconsin State Laboratory of Hygiene and forwarded to the Centers for Disease Control and Prevention (CDC) for confirmatory testing. Patients with laboratory confirmed POWV infections were interviewed to obtain demographic, clinical, and epidemiologic information.POWV infections were confirmed in 3 adult Wisconsin residents in 2003, 2006, and 2007; illness onsets occurred during May and June. Two patients were hospitalized and all survived. One patient had a dual infection with POWV and Anaplasma phaghocytophilum. Specimens from all 3 patients were initially reported as positive for IgM antibody to either St Louis encephalitis or California serogroup viruses; POWV-specific antibody was detected during confirmatory testing at the CDC. Each patient had exposures to known or likely tick habitats in different counties within 30 days before illness onset.These are the first diagnosed human POWV infections in Wisconsin. Because all 3 patients were initially identified as having other arboviral infections using commercial screening kits, routine confirmatory testing is essential for proper diagnosis of most arboviral infections. Wisconsin residents should be educated regarding risks of acquiring and ways to prevent POWV infection and other tickborne diseases when spending time outdoors.