WU and KI polyomaviruses were discovered in 2007 in samples of respiratory secretions of children with acute respiratory symptoms. Seroepidemiologic studies have shown that these viruses are widely distributed throughout the world, but their incidence in Spain has not been determined. In transplant patients, early detection and treatment of viral infections may influence prognosis and survival, because they are associated with increased morbidity and mortality, including graft failure.We aimed to determine the prevalence and clinical characteristics of WU and KI polyomaviruses among patients undergoing hematologic or solid organ transplant in the Hospital Marqués de Valdecilla (Santander, Spain). An in-house polymerase chain reaction with the use of specific primers was carried out in invasive lower respiratory samples from hospitalized patients with suspected respiratory infection and/or graft dysfunction and compared with asymptomatic transplant patients.Overall, we obtained 5.5% KI-positive samples and 1.4% WU-positive samples, with a higher prevalence of WU and KI polyomaviruses in the symptomatic population compared with the control group. Although the data suggest that their detection in respiratory samples is sporadic and often associated with other microorganisms, we should pay special attention to their association with cases of graft failure. Studies are needed with a larger number of samples to explore the potential clinical impact of these emerging polyomaviruses in transplant recipients.