Lower respiratory tract infections are the leading cause of death in children worldwide. Studies on the epidemiology and clinical associations of the four human non-SARS human coronaviruses (HCoVs) using sensitive polymerase chain reaction (PCR) assays are needed to evaluate the clinical significance of HCoV infections worldwide. Pediatric respiratory specimens (1,683) submitted to a diagnostic virology laboratory over a 1-year period (December 2004-November 2005) that were negative for seven respiratory viruses by conventional methods were tested for RNA of four HCoVs using sensitive RT-PCR assays. Coronavirus RNAs were detected in 84 (5.0%) specimens: HCoV-NL63 in 37 specimens, HCoV-OC43 in 34, HCoV-229E in 11, and HCoV-HKU1 in 2. The majority of HCoV infections occurred during winter months, and over 62% were in previously healthy children. Twenty-six (41%) coronavirus positive patients had evidence of a lower respiratory tract infection (LRTI), 17 (26%) presented with vomiting and/or diarrhea, and 5 (8%) presented with meningoencephalitis or seizures. Respiratory specimens from one immunocompromised patient were persistently positive for HCoV-229E RNA for 3 months. HCoV-NL63-positive patients were nearly twice as likely to be hospitalized (P = 0.02) and to have a LRTI (P = 0.04) than HCoV-OC43-positive patients. HCoVs are associated with a small, but significant number (at least 2.4% of total samples submitted), of both upper and lower respiratory tract illnesses in children in Colorado. Our data raise the possibility that HCoV may play a role in gastrointestinal and CNS disease. Additional studies are needed to investigate the potential roles of HCoVs in these diseases.