Between November 1996 and January 1997, 14 patients were diagnosed as having infection caused by adenovirus type 7 in a paediatric ward of Asahikawa Kosei Hospital. The age range of the patients was from two months to five years. Their diseases and abnormal laboratory findings were pneumonia in all 14, leukocytopenia in 10, myositis in nine, gastroenteritis in eight, encephalitis in five, liver dysfunction in three, pleuritis in two, inappropriate secretion of antidiuretic hormone syndrome in two, and thrombocytopenia in two. The infected patients, except for the first had been hospitalized in the paediatric ward for treatment of another disease and re-admitted because of high fever and coughing a few days after improvement or discharge. It is thought that the cause of the outbreak was hospital-acquired infection.