Merkel cell polyomavirus (MCPyV) has been found to be integrated monoclonally in a rare skin cancer named Merkel cell carcinoma. More recently, MCPyV has been detected in the upper respiratory tract of pediatric and adult patients. However, the mode of transmission and pathogenic role of MCPyV in the respiratory system has not been determined. In this study, MCPyV was sought in the lower respiratory tract of adult patients admitted to the hospital. MCPyV DNA was detected in 15 (17.24%) out of 87 lower respiratory tract samples. Most of the patients with MCPyV were over 50 years old. Nucleotide sequence of the t-antigen of MCPyV identified in respiratory secretions showed a homology to those found in Merkel cell carcinoma. In addition, phylogenetic analysis undertaken on the t-antigen sequences of Italian isolates and other MCPyVs identified in healthy and cancer tissues showed that all these isolates belonged to the same clade. Selective pressure analysis for the t-antigen revealed the presence of five sites under positive selection (omega = 4.3), with a posterior probabilities above 0.99. The alpha parameter of the gamma distribution was 0.01, showing that this distribution has a characteristic L-shape and suggesting a strong nucleotide substitution rate heterogeneity across sites. This study shows that MCPyV can infect the lower respiratory tract, but further investigations are needed to define its pathogenic role in respiratory diseases. J. Med. Virol. 82:505-509, 2010. (c) 2010 Wiley-Liss, Inc.