Human coronaviruses (HCoVs) are associated with a variety of clinical presentations in children, but their role in disease remains uncertain. The objective of our prospective study was to investigate HCoVs associations with various clinical presentations in hospitalized children up to 6 years of age. Children hospitalized with acute bronchiolitis (AB), acute gastroenteritis (AGE), or febrile seizures (FS), and children admitted for elective surgical procedures (healthy controls) were included in the study. In patients with AB, AGE, and FS, a nasopharyngeal (NP) swab and blood sample were obtained upon admission and the follow-up visit 14 days later, whereas in children with AGE a stool sample was also acquired upon admission; in healthy controls a NP swab and stool sample were taken upon admission. Amplification of polymerase 1b gene was used to detect HCoVs in the specimens. HCoVs-positive specimens were also examined for the presence of several other viruses. HCoVs were most often detected in children with FS (19/192, 9.9%, 95% CI: 6-15%), followed by children with AGE (19/218, 8.7%, 95% CI: 5.3-13.3%) and AB (20/308, 6.5%, 95% CI: 4.0-9.8%). The presence of other viruses was a common finding, most frequent in the group of children with AB (19/20, 95%, 95% CI: 75.1-99.8%), followed by FS (10/19, 52.6%, 95% CI: 28.9-75.6%) and AGE (7/19, 36.8%, 95% CI: 16.3-61.6%). In healthy control children HCoVs were detected in 3/156 (1.9%, 95% CI: 0.4-5.5%) NP swabs and 1/150 (0.7%, 95% CI: 0.02-3.3%) stool samples. It seems that an etiological role of HCoVs is most likely in children with FS, considering that they had a higher proportion of positive HCoVs results than patients with AB and those with AGE, and had the highest viral load; however, the co-detection of other viruses was 52.6%. ClinicalTrials.gov NCT00987519.