To the Editors: A 9-month-old previously healthy girl was admitted to the KKR Sapporo Medical Center with 1-day history of nasal discharge and high fever. A chest roentgenogram showed lung hyperinflation with increased vascular markings. She was diagnosed with bronchitis and was treated with hydration and inhalation therapy with a β2-stimulant. She was discharged from the hospital after 3 days. The nasopharyngeal (NP) sample was subjected to reverse transcription-polymerase chain reaction (RT-PCR) screening for 14 viral pathogens (respiratory syncytial virus (RSV), human metapneumovirus, human rhinovirus, human bocavirus, parainfluenza viruses 1–3, influenza A virus, influenza B virus, human enterovirus, human coronaviruses, adenovirus, KI polyomavirus, and WU polyomavirus [WUPyV]) as previously described.1 RSV and WUPyV were detected. Serum samples were studied for WUPyV DNA using a real-time PCR assay in the acute phase (4 days after onset) and the convalescent phase (13 days after onset), and IgG antibody titers to virus protein 1 (VP1; a capsid protein of WUPyV) were measured using an immunofluorescence assay as previously described.1,2 WUPyV DNA was detected in the acute phase (9.6 × 102 copies/mL) but not in the convalescent phase (<5.0 × 10 copies/mL). The titers of the IgG antibody against WUPyV VP1 increased from <1:10 (acute phase) to 1:40 (convalescent phase). In 2007, a novel virus, WUPyV, was cloned from respiratory tract samples in the United States.3 Although the WUPyV genome was detected in 1.2% to 16.4% of NP samples in patients with respiratory tract infections,1,3 it was not clear whether WUPyV caused infection or was an innocent bystander.4 Other viruses (RSV, human metapneumovirus, human rhinovirus, human bocavirus, and parainfluenza virus 1) were simultaneously detected in 44.7% of the WUPyV-positive NP samples, which complicated understanding the role of WUPyV in respiratory tract infections.1 Clinical findings of our case were indistinguishable from those for patients with RSV-positive bronchitis, but the present study demonstrated infection with WUPyV in a patient with bronchitis by genetic and serologic testing. Both detection of the WUPyV genome in serum samples by real-time PCR and measurement of antibody titers against WUPyV VP1 by an immunofluorescence assay are useful for analysis of the pathogenesis of WUPyV infection. Shinobu Teramoto, MD Department of Pediatrics Hokkaido University Graduate School of Medicine Naoko Koseki, MD Mikio Yoshioka, MD, PhD Yukiko Matsunami, MD Noriko Yanazume, MD Mitsuru Nawate, MD Takaaki Shikano, MD, PhD Yutaka Takahashi, MD, PhD Department of Pediatrics KKR Sapporo Medical Center Sapporo, Japan Hideaki Kikuta, MD, PhD Pediatric Clinic Touei Hospital Sapporo, Japan Nobuhisa Ishiguro, MD, PhD Department of Pediatrics Hokkaido University Graduate School of Medicine Sapporo, Japan