Hand washing with soap and water can prevent the spread of diarrhoeal diseases in areas where comparatively costly interventions, such as supply of safe water and improved sanitation, are not possible. In this study, the practice of hand washing with soap and water was instituted in a periurban slum of Dhaka city, and the surveillance for diarrhoea sustained for a one-year period. Rates of primary and secondary attacks were compared to those of a non-intervention area similar in age structure, economic status, education, and other relevant variables. Rectal swabs of cases and contacts established aetiologies. There was a large (2.6 fold) reduction in diarrhoeal episodes in the intervention area during the observation period. Rates of bacterial pathogens were also lower in the intervention area. Significant reduction in diarrhoeal incidences was observed in all age groups for all pathogens except for rotavirus. These observations if implemented as health policy could reduce the spread of diarrhoeal diseases at low cost in high risk areas.