Diarrhea and malnutrition are common in young children in developing countries and a reciprocal relationship has been postulated with diarrhea leading to malnutrition and malnutrition predisposing to diarrhea. To investigate the importance of malnutrition as a determining factor in diarrheal illnesses, data were analyzed from a longitudinal community-based study done in rural Bangladesh. Children classified by nutritional status according to a variety of anthropometric indicators were prospectively evaluated for incidence, duration, and etiology of diarrhea. Children with low weight for length had longer durations of diarrhea than better nourished children; however, children of differing nutritional status had similar diarrheal incidences. The duration of diarrhea, including that associated with enterotoxigenic Escherichia coli and Shigella, increased progressively as nutritional status indicators worsened. These results suggest that nutritional interventions alone are unlikely to reduce the high incidence of diarrhea, but that efforts to improve nutritional status may have a beneficial effect on the duration of diarrhea and its unfavorable nutritional consequences.