PAO was responsible for the experiment, data analysis and drafted the manuscript. KKL conducted the real-time RT-PCR analysis, and contributed throughout the experimental process. AEOJ constructed the qPCR assays for two of the genes. TON provided the cDNA from the smoltification experiment. IH participated as a supervisor in the study design, analyses and writing.  Relevance of human metapneumovirus in exacerbations of COPD BACKGROUND AND METHODS: Human metapneumovirus (hMPV) is a recently discovered respiratory virus associated with bronchiolitis, pneumonia, croup and exacerbations of asthma. Since respiratory viruses are frequently detected in patients with acute exacerbations of COPD (AE-COPD) it was our aim to investigate the frequency of hMPV detection in a prospective cohort of hospitalized patients with AE-COPD compared to patients with stable COPD and to smokers without by means of quantitative real-time RT-PCR. RESULTS: We analysed nasal lavage and induced sputum of 130 patients with AE-COPD, 65 patients with stable COPD and 34 smokers without COPD. HMPV was detected in 3/130 (2.3%) AE-COPD patients with a mean of 6.5 × 10(5 )viral copies/ml in nasal lavage and 1.88 × 10(5 )viral copies/ml in induced sputum. It was not found in patients with stable COPD or smokers without COPD. CONCLUSION: HMPV is only found in a very small number of patients with AE-COPD. However it should be considered as a further possible viral trigger of AE-COPD because asymptomatic carriage is unlikely. Respiratory viruses play an important role in exacerbations of COPD and this has been increasingly recognised since the application of molecular detection methods [1, 2] . The most prevalent viruses detected by polymerase chain reaction so far were respiratory syncytial virus (RSV), Influenza A, Rhinovirus and Parainfluenza 3. Human metapneumovirus (hMPV) is a recently discovered respiratory virus first isolated from a dutch child with lower respiratory tract infection (LRTI) [3] . World wide distribution is probable since it has been isolated in North HMPV has been recognized as a member of the Paramyxoviridae like RSV and it is not only associated with bronchiolitis in most cases, but also with pneumonia, croup and exacerbations of asthma [14, 15] , diseases which share some features with COPD. Up to date reports about hMPV in adults are scarce. In a general Canadian population 14.8% of patients of all age groups with acute respiratory tract infections were hMPV positive. Thirty-three percent of hMPV-infected patients were hospitalized and the hospitalization rates were significantly higher among patients below 5 years and those over 50 years of age [16] . In another prospective cohort of adults, hMPV was detected in 4.5% of all illnesses but also in 4.1% of asymptomatic subjects. HMPV was most prevalent in young adults with children and in frail elderly [17] . HMPV infection can be severe since the virus was isolated from the lungs from a previously healthy man who died from acute pneumonia [18] . The role of hMPV in acute exacerbations of COPD (AE-COPD) has been studied recently in outpatients and only low frequencies have been observed [17, 19] . Up to now the prevalence of hMPV in patients hospitalized with AE-COPD is unknown. Our aim was therefore to investigate the frequency of detection of hMPV in a prospective cohort of hospitalized patients with AE-COPD and to compare these results to patients with stable COPD and to smokers without COPD.