Group A rotaviruses are the major cause of diarrhea in young children worldwide. From March 2001 to April 2002, 836 children less than 5 years of age were investigated in Hanoi, Vietnam. This included 587 children with diarrhea and 249 age-matched controls. Group A rotavirus was identified in 46.7% of the children with diarrhea and 3.6% of the controls, which was a significant difference. Within the diarrhea group, the highest prevalence was seen in children from 13 to 24 months of age, and the prevalence was higher in males than in females. The symptoms of acute diarrhea caused by rotavirus were watery diarrhea, vomiting, fever, and dehydration. A higher prevalence of rotavirus detection was obtained for children who had all of these symptoms, followed by those who had diarrhea with vomiting-dehydration, fever-dehydration, and dehydration. The high rates occurred from September to December, although the infection was encountered all year round. In 58 patients (21.2% of the rotavirus-infected children), rotavirus infection was detected in association with either diarrheagenic Escherichia coli or Shigella spp. The most frequent combinations were rotavirus-enteroaggregative E. coli and rotavirus-enteropathogenic E. coli. At least one enteropathogen was identified from about 64% percent of the samples. The bacterial infection may not have given rise to clinical symptoms of such severity. The present study demonstrates the burden of rotavirus diarrhea in Hanoi, Vietnam. Continuous surveillance of diarrhea caused by rotavirus in young children would play an important role in diagnosis, treatment, and prophylaxis in order to improve the health of children in Vietnam.