Outbreaks of calicivirus gastroenteritis are common during the winter season, especially among elderly residents in institutions. The outbreaks are often difficult to control in institutions and may occasionally result in closure of wards. Reliable diagnostic methods are therefore important. We have studied the prevalence of calicivirus in samples sent to our laboratory for calicivirus detection during the winter season 2001 to 2002.The presence of calicivirus was studied by nested RT-PCR. A total of 117 faecal samples obtained from patients with suspected calicivirus gastroenteritis were examined.Sixteen outbreaks of acute gastroenteritis were observed during the study period. While calicivirus was confirmed in nine of the outbreaks, the virus was detected in only 25 of the 117 faecal samples analyzed (21%). An increased detection rate was observed when faecal samples were collected during the first two days after onset of symptoms.Appropriate surveillance systems are required in order to control calicivirus gastroenteritis in institutions. Microbiological laboratories should be aware of the importance of collecting specimens during the acute phase (i.e., within 48-72 hours after onset) of illness.