Diarrhea and malnutrition, alone or together, constitute major causes of morbidity and mortality among children throughout the tropical world. Data from northeast Brazil, taken with numerous other studies, clearly show that diarrhea is both a cause and an effect of malnutrition. Diarrheal illnesses impair weight as well as height gains, with the greatest effects being seen with recurrent illnesses, which reduce the critical catch-up growth that otherwise occurs after diarrheal illnesses or severe malnutrition. Malnutrition (whether assessed by impaired weight or height for age) leads to increased frequencies and durations of diarrheal illnesses, with a 37% increase in frequency and a 73% increase in duration accounting for a doubling of the diarrhea burden (days of diarrhea) in malnourished children. A multi-pronged approach focusing on those with prolonged diarrhea and severe malnutrition is suggested.