Human metapneumovirus (hMPV) was newly discovered as a pathogen in 2001 and is thought to be associated with respiratory disease. To elucidate the prevalence and clinical significance of hMPV among children, we investigated the positive cases of hMPV-RNA by reverse transcription-polymerase chain reaction (RT-PCR) in their nasopharyngeal specimens collected from January to August 2003 in Hiroshima Prefecture, Japan. Our prospective study revealed 77 hMPV-positive cases among 377 children with acute respiratory diseases. Clinical diagnoses of 77 hMPV-positive cases were as follows; bronchitis (33.8%), pneumonia (24.7%), acute respiratory illness (19.5%), asthmatic bronchitis (11.7%) and bronchiolitis (5.2%). The most common symptoms were cough (97.4%), high fever (94.8%) and rhinorrhea (76.6%). Most of the hMPV-positive cases were identified in the spring (between March and May), indicating the presence of an epidemic of hMPV infection in Hiroshima Prefecture. Phylogenetic analysis of the amplified F gene of hMPV isolates revealed that hMPV strains were divided into two genotypes and that their simultaneous circulation occurred within the same epidemic area of Hiroshima Prefecture.