PIP: To determine the magnitude of morbidity related to diarrhea among children under 5 years of age, a study was carried out in 4 slum areas of Delhi in May-June 1984. Of the 3645 children surveyed, 963 (26.4%) had suffered from 1 or more new episodes of diarrhea in the 2 weeks preceding the survey. This suggests a mean annual incidence of diarrhea in the slums of Delhi of 7.9 episodes/child, with an average duration of 3.9 days/episode. The mean episodes/child/year ranged from a low of 4.1 in Ballimaran to a high of 11.9 in the Seelampur slum. The incidence of diarrhea was highest in chldren 7-12 months of age (13.6 episodes/year). In 24.2% of the diarrhea episodes, the families sought no medical help. 54.5% of cases were treated by private practitioners, 21% were seen at hospitals, and 0.3% were treated with home remedies. Only 20.2% of mothers of children with diarrhea were familiar with oral rehydration therapy, despite the easy accessibility of information about this treatment in the slum areas. Among the mothers with knowledge of oral rehydration, 47.9% used it for all diarrhea episodes, 33.0% used it for selected episodes, and 19.1% had never used it. A higher incidence of diarrhea was found among children living in homes where unfiltered water from handpumps was used for drinking or where residents defecated in open fields. Children with grade 2 or 3 malnutrition also showed a higher incidence of diarrhea. Among the socioeconomic parameters, diarrhea was positively associated with low female income and illiteracy on the part of the mother. Finally, infants who were exclusively breastfed had a lower incidence of diarrhea than those who were receiving supplementary feedings.