Rhinoviruses (HRVs) were isolated from 307 children (7.1%) in the virological surveillance of 4334 children with acute respiratory tract illnesses in Morioka, Japan (September 1973-December 1983). Although HRVs were isolated throughout the year, frequency of HRV infection was significantly higher (p less than 0.001) during the April-November (233/2853; 8.2%) than during the December-March (47/1481; 5.0%). There were two peaks of incidence in May (9.5%) and September (9.1%). During the May-September, the rate of HRV infection was higher in patients under the age of 11 months than the next higher group of 1-2 years old (p less than 0.001). The incidence decreased with increasing age. The illnesses of HRV infection were analysed in 294 patients, except one patient who had symptoms of measles, from whom HRV was isolated singly. Although HRV-associated illnesses were generally mild (57.5%). Upper respiratory tract illnesses (URTIs) with fever were found in 22.1% and lower respiratory tract illnesses (LRTIs) in 20.4% of these. The rate of LRTI was higher during the epidemic period (April-September) than other periods (p less than 0.02). Major symptoms of HRV-associated illnesses observed were sore throat (87.4%), cough (84.0%), and nasal obstruction and/or discharge (72.8%). Wheezing was observed in 21.8% of these. From 19 (21.8%) of 47 patients clinically diagnosed as asthmatic bronchitis in this survey, viruses were isolated. HRV was detected most frequently in 12.8% of these patients, followed by respiratory syncytial virus (RSV, 6.4%) and adenovirus (2.1%). HRV- and RSV-associated asthmatic bronchitis were observed during April-September and November-February, respectively. Viral dual infections were detected in total 20 cases included 12 HRV-associated cases. In no case was the illness of greater severity than might have been caused by either agent acting singly.