The human caliciviruses norovirus and sapovirus are leading causes of acute, non-bacterial gastroenteritis. In contrast to norovirus, sapovirus is known to give infections mainly in infants and young children. We describe a nosocomial outbreak of gastroenteritis associated with sapovirus involving 23 adult patients and medical staff. The mean age of the patients and medical staff was 52 y and the major signs and symptoms were nausea, diarrhoea, vomiting, abdominal cramp, headache, myalgia and fever. More patients had diarrhoea (72%) than vomiting (56%) and the mean duration of symptoms was 6 d. A secondary attack rate of 45% was seen affecting in all 10 persons with a mean age of 29 y. Sequences of the capsid gene revealed a 97% nucleotide homology to the sapovirus genogroup IV reference strain Chiba/000671T/1999. This is one of the first reported nosocomial outbreaks of sapovirus infection among adults and shows that a diagnostic test for sapovirus should be included in investigation of gastroenteritis among adults.