This is a systematic review of studies using RT–PCR to diagnose norovirus (NoV) infections in patients with diarrhoea. NoVs accounted for 12% of severe gastroenteritis cases among children <5 years of age and 12% of mild and moderate diarrhoea cases among persons of all ages. Of 19 studies among children <5 years of age, 7 were in developing countries where pooled prevalence of severe NoV disease (12%) was comparable to that for industrialised countries (12%). It was estimated that each year NoVs cause 64,000 episodes of diarrhoea requiring hospitalisation and 900,000 clinic visits among children in industrialised countries, and up to 200,000 deaths of children <5 years of age in developing countries. Emerg Infect Dis. 2008;14(8):1224-31.