The objective of this study was to investigate risk factors for norovirus-associated infectious intestinal disease (IID) and asymptomatic norovirus infection. Individuals with IID and healthy controls were recruited in a community-based study in England (1993-1996). This is the first risk-factor study to use viral load measurements, generated by real-time RT-PCR, to identify cases of norovirus-associated IID and asymptomatic infections. Using multivariable logistic regression the main risk factor identified for norovirus-associated IID was contact with a person with IID symptoms. Infectious contacts accounted for 54% of norovirus cases in young children and 39% of norovirus cases in older children and adults. For young children, contacts outside the household presented the highest risk; for older children and adults, the highest risk was associated with child contacts inside the household. Foreign travel and consumption of shellfish increased the risk of norovirus-associated IID. Lifestyle and dietary factors were associated with a decreased risk of both norovirus-associated IID and asymptomatic infection. No risk factors were identified for asymptomatic norovirus infection.