PIP: Diarrheal diseases pose a major threat to the health of people living in developing countries, particularly where there is poverty and a poor environment. The effect of handwashing in reducing the incidence of diarrhea in day-care centers has been reported recently. The present study is a prospective community based study conducted to evaluate the effect of handwashing on the incidence of diarrheal disease in Calcutta slums. Handwashing was implemented in 1 slum and the other nearby slum, without handwashing program, served as control. The incidence of watery diarrhea during the 13 month period, between the study and control groups was not significantly different. However, the incidence of dysentery in individuals above 5 years of age in the control group was significantly higher as compared to those in the study group (p 0.05). The results of this study indicate that handwashing with soap may reduce the incidence of dysenteric cases in the community by interruption of transmission of the pathogens from 1 person to the other.