Objective.Norovirus(NV) is an etiologic agent of outstanding importance that can cause severe epidemic gastroenteritis in day-care centers,schools,nursing homes,and hospitals.Therefore NV requires foremost attention as a pathogen responsible for epidemics of gastroenteritis in immunocompromised inpatients.In this study,a NV outbreak in a pediatric oncology unit is described and the consequences for this high-risk population are discussed.Material and methods.Stool and vomitus samples from 11 patients were tested for NV and other relevant viruses during the outbreak by reverse transcriptase-polymerase chain reaction(RT-PCR) and/or enzyme-linked immunosorbent assay(ELISA) (whenever an appropriate ELISA was available) .Norwalk virus PCR amplifications were sequenced and phylogenetic analysis was performed.Results.The index patient and the chain of infection were identified.Follow-up investigation surprisingly demonstrated viral shedding for a maximum of 140 days(median 23 days) .Three patients experienced severe or life-threatening symptoms,probably related to NV infection.Conclusions.In the event of an outbreak of gastroenteritis(involving two or more symptomatic patients) in a pediatric oncology unit,the search for NV in stool or vomitus specimens should be initiated in good time.As long as the data are limited regarding whether a detectable viral antigen or RNA in stools represents an infectious virus,patients have to be isolated as long as the diagnostic assays remain positive.During the acute phase of the illness,health-care workers should wear masks in addition to practicing meticulous hand hygiene with a disinfectant of proven activity against NV.Pediatric oncology patients must be closely monitored during follow-up investigations as they may shed the virus for months.There is some evidence from the outbreak described here that those patients face a greater risk of severe NV-related complications. ObjectivesNorovirus (NV) is an etiologic agent of outstanding importance that can cause severe epidemic gastroenteritis in day-care centers, schools, nursing homes, and hospitals.Therefore NV requires foremost attention as a pathogen responsible for epidemics of gastroenteritis in immunocompromised inpatients. In this study, a NV outbreak in a pediatric oncology unit is described and the consequences for this high-risk population are discussed. Material and methods. Tomato and vomitus samples from 11 patients were tested for NV and other relevant viruses during the outbreak by reverse transcriptase-polymerase chain reaction (RT-PCR) and / or enzyme-linked immunosorbent assay (ELISA) whenever an appropriate ELISA was available. Norwalk virus PCR amplifications were sequenced and phylogenetic analysis was performed. Results. The index patient and the chain of infection were identified. Frolow-up investigation surprisingly demonstrated viral shedding for a maximum of 140 days (median 23 days) .Three pat炊 experienced severe or life-threatening symptoms, probably related to NV infection. Conclusions. the event of an outbreak of gastroenteritis (involving two or more symptomatic patients) in a pediatric oncology unit, the search for NV in stool or vomitus specimens should be initiated in good time. As long as the data are limited or whether in detectable viral antigen or RNA in stools represents an infectious virus, patients have to be isolated as long as the diagnostic assays remain positive. During the acute phase of the illness, health -care workers should wear masks in addition to practicing meticulous hand hygiene with a disinfectant of proven activity against NV. Pediatric oncology patients must be closely monitored during follow-up investigations as they may shed the virus for months. There is some evidence from the outbreak described here that those patients face a greater risk of severe NV-related complications.