e Noroviruses (NoVs) have emerged as the leading cause of acute viral gastroenteritis (GE) in humans. Although diagnostic facilities have greatly improved, significant underdiagnosis of NoV in hospitals may still occur, thereby increasing clinical burden and nosocomial spread. We evaluated the underdiagnosis of sporadic NoV infections in a tertiary care hospital and estimated its clinical impact. From December 2008 until July 2009, fecal samples specifically referred for bacterial but not viral examination were retrospectively tested for NoV by real-time PCR. The clinical and virological data from patients with undiagnosed NoV infection (missed patients) were evaluated and compared with those from patients with recognized NoV. During the study period, 45 patients with undiagnosed NoV were detected, whereas 50 patients were regularly diagnosed. The missed NoV cases were more frequently adults than children (80% versus 46%; P < 0.001). The viral load levels did not differ between the diagnosed and missed patients, but missed patients more frequently presented without diarrhea (20% versus 4%; P < 0.07). The newly admitted missed NoV cases with GE underwent more diagnostic imaging (24% versus 4%; P < 0.01) and tended to be hospitalized longer. When missed-NoV patients were included, the number of nosocomial clusters doubled and missed patients were index cases in 5 of the 6 clusters. These data indicate that NoV infections are frequently missed despite routine laboratory testing and demonstrate that underdiagnosis of NoV patients is associated with costly abdominal imaging and nosocomial clustering. Awareness of NoV infection in adult patients and education about the importance of viral GE should be increased. N oroviruses (NoVs) have emerged as the one of the most important pathogens causing acute gastroenteritis (GE) in children and adults (8,12). Nursing homes and hospitals are widely confronted with NoV outbreaks. Additionally, isolated (sporadic) cases of NoV frequently occur, but their incidence and clinical impact in hospitals have not been studied systematically (2, 17). Sporadic cases of NoV may result both from community-acquired infections in newly admitted patients and from nosocomial transmissions between patients, personnel, or visitors (9). Although sensitive commercial and homemade diagnostic assays for NoV have become widely available, sporadic NoV infections in hospitalized patients remain underdiagnosed, increasing the clinical burden and potential for nosocomial spread (1, 3, 21). Underrecognition of NoV may result in the individual patient undergoing more diagnostic procedures and may increase the influx of infectious patients into hospital wards, where they may trigger outbreaks (5, 14). Underdiagnosis of NoV may result from a referral bias, as well as from suboptimal laboratory facilities and inadequate specimen collection. This bias may occur when physicians selectively refer GE patients for bacteriological or parasitological testing but not virological testing. In the present study, we prosp...