Background: Acute diarrheal disease among children younger than 5 years old remains a major cause of morbidity and mortality worldwide. Severe infectious diarrhea in children occurs most frequently under circumstances of poor environmental sanitation and hygiene, inadequate water supplies, and poverty. In Vietnam, the control of diarrhoeal disease (CDD), including promotion of breast-feeding, oral rehydration therapy and specific health education is a part of national strategies aiming to improve the quality of life and reduce the burdens caused by diseases. Despite this fact, diarrheal disease is still the second leading cause of infectious morbidity and mortality in children as well as in adults in Vietnam. The local epidemiology of diarrhea in most rural areas of Vietnam has not been researched thoroughly. In addition, most studies in Vietnam have focused on a specific pathogen rather than identifying the most common pathogens of diarrhea among children in rural areas. Better understand the local epidemiology of diarrhoeal disease could be a valuable contribution to the development of public health prevention. We therefore conducted a study in Dong Anh Hospital in order to identify risk factors for diarrhea among children less than five years of age in this area.Objectives: the study aimed to identify the most common causes of and risk factors for diarrheal disease among children aged less than five years admitted to Dong Anh Hospital, Hanoi.Method and materials: a hospital-based case-control study was performed. A case was defined as a child less than 5 years of age having three or more loose, liquid, or watery stools or at least one bloody loose stool within the last 24 hours. Accordingly, all cases admitted to Dong Anh Hospital between July and December 2005 which fulfilled the inclusion criteria were recruited into the study. Controls were non-diarrheal patients matched for sex and age. Face-to-face interviews based on the questionnaire were conducted with mothers on the day of admission. Stool samples were collected from all cases immediately after their admission, and were then processed for bacterial, parasitological, and viral studies.Results: A total of 600 study subjects, including 200 cases and 400 controls, were recruited into the study. Cases were mostly children less than 24 months of age. The number of boys was higher than girls in nearly all age groups.In multivariate analysis, using conditional logistic regression, some factors remained independently associated with the risk of diarrhea, namely the child having sibling(s) (OR=1.9; 95% CI 1.2 - 3.2); irregular latrine cleaning (OR=4.4; 95% CI 2.4 - 8.1); latrine-sharing among more than 5 people (OR=2.8; 95% CI 1.3 - 6.2); irregular hand washing by mothers after going to toilet (OR=4.5; 95% CI 2.1 - 9.5); no hand-washing by mothers before feeding children (OR=9.4; 95% CI 2.3 - 37.6); unsafe storage of food for later use (OR=3.4; 95% CI 2.0 - 5.7); irregular kitchen cleaning(OR=4.3; 95% CI 2.5 - 7.4); and infrequent cleaning/emptying of storage container before refilling it with fresh water (OR=7.7; 95% CI 4.4 - 13.5).Among 200 stool samples collected in the study, we detected 54 cases positive to entero pathogenic Escherichia coli (EPEC), 50 cases to rotavirus and 8 cases to Shigella spp. Co-infecton of rotavirus-EPEC was found in 13 cases, and rotavirus-Shigella in one case. Infection with Entamoeba hystolytica was also detected in 23 cases. Conclusion: From this study we identified the risk factors of diarrhea to be irregular hand-washing by mothers after going to toilet, no hand-washing by mothers before feeding children, the child having sibling, unsafe storage of food for later use, irregular kitchen cleaning, infrequent cleaning/emptying of storage container before refilling it with fresh water and irregular latrine cleaning, latrine-sharing among more than 5 people. EPEC, Rotavirus and Shigella spp. are found to be common pathogens for diarrhea among children admitted to in Dong Anh Hospital.From these findings we suggest that encouraging mothers, through education, to wash their hands before feeding their children or after going to toilet should be a priority. Improving hygienic practice in the community through education programmes participated by volunteers, mothers' support groups, health workers, mass media; building kindergartens in all villages; implementing community IMCI (Integrated Management of Childhood Illness); and establishing intersectoral collaboration are the main methods we wish to recommend in order to improve public awareness of diarrhea, eventually aiming to reduce burden caused by diarrhea among children less than five years of age in the district.Key words: diarrheal disease; risk factors; epidemiology; pathogens; children under five years of age; rural areas; Vietnam.