OBJECTIVE To determine the efficacy of zinc-fortified oral rehydration salts solution (ORS) in comparison to ORS without zinc in 6- to 35-month-old urban children with acute diarrhea not sick enough to be hospitalized. METHODS Double-blind, randomized, controlled trial. METHODS Children (n = 1219) with acute diarrhea were randomly assigned to one of 3 groups. The first group received a zinc syrup (15 mg zinc to 6- to 11-month-old children and 30 mg to 12- to 35-month-old children), the second group received zinc premixed with ORS (40 mg/L), and the control children received ORS only. Households were visited twice weekly until recovery. RESULTS The total number of stools was lower in the zinc-ORS group (rate ratio, 0.83; 95% CI, 0.71-0.96), as was the proportion of children with watery stools (odds ratio, 0.61; 95% CI, 0.39-0.95), compared with the control group; there was no significant effect on diarrheal duration. ORS intake and proportion of children with vomiting were not significantly different between the zinc-ORS and control groups. The zinc syrup group had lower diarrheal duration (relative hazards, 0.89; 95% CI, 0.80-0.99) and total stools (rate ratio, 0.73; 95% CI, 0.70-0.77) than control children. CONCLUSIONS Zinc-ORS was moderately efficacious in reducing the severity of acute diarrhea without increasing vomiting or reducing ORS intake.