An educational intervention was designed to improve three water-sanitation behaviors empirically shown to be associated with high rates of childhood diarrhea in Dhaka, Bangladesh: lack of handwashing before preparing food, open defecation by children in the family compound, and inattention to proper disposal of garbage and feces, increasing the opportunity for young children to place waste products in their mouth. Fifty-one communities, each comprising 38 families, were randomized either to receive (n = 25) or not to receive (n = 26) the intervention. During the six months after the intervention, the rate of diarrhea (per 100 person-weeks) in children under six years of age was 4.3 in the intervention communities and 5.8 in the control communities (26% protective efficacy; p less than 0.0001). A corresponding improvement in handwashing practices before preparing food was noted, although no improvement was observed for defecation and waste disposal practices. These data suggest that educational interventions for water-sanitation practices can have an important beneficial effect upon childhood diarrhea in developing countries, particularly when the interventions are designed in a simple way to promote naturally occurring salutory behaviors that are empirically associated with lower rates of childhood diarrhea.