The clinical efficacy of a diluted oral rehydration salts (ORS) solution was compared in a pilot study with that of intravenous (i.v.) therapy and of standard World Health Organization (WHO)/United Nations Childrens Fund (UNICEF) ORS solution in children with acute diarrhea. Sixty-one boys aged 3 to 24 months, admitted to hospital with acute diarrhea and signs of dehydration, were randomly assigned to groups receiving standard ORS solution, diluted ORS solution, or i.v. therapy. In children treated with standard ORS solution and small amounts of plain water, the total fluid intake was 25-39% greater, the stool output was 58-77% greater (p < 0.01), and the duration of diarrhea was 30-55% greater than in the other treatment groups. Intake of plain water, taken separately or added to the ORS solution, was greater in children given diluted ORS solution (73 +/- 23 ml/kg) than in those given standard ORS solution (21 +/- 32 ml/kg) (p < 0.001). The mean serum sodium concentration increased by 2.2 mEq/L in children given standard ORS solution, whereas it decreased by 2.9 mEq/L in those given diluted ORS solution. This study shows that some children develop worsening diarrhea and increasing serum sodium concentrations when treated with standard ORS solution and given only small amounts of plain water. This is probably caused by the slight hypertonicity of standard ORS solution combined with transient partial glucose malabsorption. This can be avoided if water, breast milk, or another low-solute drink is given liberally during maintenance therapy with ORS solution, as recommended by the WHO.