Noroviruses are generally believed to cause relatively mild gastroenteritis of short duration in otherwise healthy adults. However, outbreaks in health care settings are common and affect vulnerable populations. During 2002-2003, a total of 4 major hospitals, 11 community hospitals, and 135 nursing homes in the county of Avon, England, were prospectively monitored for outbreaks of gastroenteritis. For 482 hospital staff, 166 nursing home staff, and 266 nursing home residents, the median duration of norovirus gastroenteritis was 2 days, with 75% achieving complete recovery within 3 days. The median duration of norovirus gastroenteritis for 730 hospital patients was 3 days (75% of the patients achieved complete recovery within 5 days), which was significantly longer than that for all other groups (P<.001). Therefore, infection in hospitalized persons may be more severe than that in other groups in the community at large. This increased duration of acute illness should be considered when implementing measures to prevent transmission in hospital settings.