Oral rehydration therapy (ORT) has become the cornerstone of most programmes of diarrhoea control, and is highly effective in preventing death from dehydration caused by acute watery episodes of diarrhoea. The effectiveness of ORT in reducing mortality from chronic or dysenteric diarrhoeas is believed to be low and, in addition, ORT can be expected to have little or no impact on diarrhoea morbidity rates. In view of this, ORT should be complemented by other interventions designed to avert mortality not averted by ORT, and to reduce morbidity rates. The Diarrhoeal Diseases Control Programme of WHO has initiated a systematic study of the effectiveness and cost of interventions that are potentially useful in diarrhoea control. This paper summarizes the findings of this study, concentrating on those interventions for which the evidence for high effectiveness and feasibility is strong. These are: promotion of breast feeding, weaning education, measles immunization, improving water supply and sanitation, promotion of hygiene, and, when the new vaccines are available, rotavirus, and possibly cholera, immunization. Estimates are presented of the cost-effectiveness of these interventions in reducing diarrhoea morbidity and mortality rates among children under 5 years of age.