A virological surveillance of acute respiratory tract illnesses (RTI) of children in Morioka, Japan, was maintained from September 1973 to December 1983. Nasal and throat swabs were collected from 4,334 children with RTI. These patients consisted of 3,500 children (80.8%) with upper RTI (URTI) and 834 children (16.2%) with lower RTI. When these patients with URTI were classified by maximum temperature recorded into 4 groups (less than or equal to 37.0 degrees C, 37.1-37.9 degrees C, 38.0-38.4 degrees C, and greater than or equal to 38.5 degrees C), the number of patients in each group was 1,909 (44.0% of all patients), 702 (16.2%), 378 (8.7%), and 512 (11.8%), respectively. The viruses were recovered from 932 patients (21.5%). The most frequently recovered virus was rhinovirus (HRV; 31.7% of positive patients). The other common viruses were respiratory syncytial virus (RSV; 15.3% of positives), enteroviruses (13.9%), adenoviruses (13.6%), and influenza viruses (11.1%). Coronaviruses (HCV) were isolated from 2 patients and dual infections were detected in 20 patients (2.1% of positives). HRVs were isolated from 8.9% of afebrile URTI, in addition to 7.4% of bronchitis and 12.8% of asthmatic bronchitis. RSVs were mainly isolated in the winter, but some of them were isolated in the summer of 1974 and 1975. HCVs were isolated from 2 patients with afebrile URTI in the spring of 1979. Consistent patterns have been observed in clinical manifestation, seasonal occurrence, and age distribution of infections; we indicated that HRV was not only most important pathogen of URTI, but also it was important in bronchitis and asthmatic bronchitis of children.