Studies have reported the presence of KI polyomavirus (KIPyV) and WU polyomavirus (WUPyV) in respiratory secretions of young patients. So far, evidence has not supported a link between infections with either virus and respiratory tract disease; however, there has not been a large comparison of KIPyV-infected patients to age-matched patient groups.A retrospective study comparing clinical aspects of KIPyV-positive patients with respiratory syncytial virus (RSV)-positive, WUPyV-positive, and respiratory-virus negative patients. Using real-time polymerase chain reaction, 2599 respiratory samples from patients ranging from 1 day to 88 years of age were tested for KIPyV. Electronic medical records were reviewed for 65 cases, for a comparison group consisting of 195 patients negative for common respiratory viruses, and for 56 WUPyV-positive patients drawn from the same population. Twelve patients testing positive for KIPyV as the sole pathogen were matched to 36 RSV-positive patients and clinical features of both groups were compared.Seventy-two (2.8%) respiratory samples were positive for KIPyV. Another virus was detected in 71% of the KIPyV-positive samples. Analysis showed no statistically significant differences in clinical manifestations between KIPyV-positive patients and patients negative for common respiratory viruses, however, clinical characteristics of KIPyV-positive patients were less severe than those of patients positive for RSV. KIPyVpositive patients >or=3 years of age were usually immunocompromised in contrast to the younger children with KIPyV.This study did not demonstrate a link between KIPyV infection and symptomatic respiratory disease. Patients positive for KIPyV exhibited less severe clinical symptoms than patients positive for RSV.