One hundred thirty children (65-95 mo old) from a low-socioeconomic neighborhood of Guatemala City participated in a randomized, double-blind, controlled trial of zinc supplementation. One group received 10 mg Zn/d (n = 65) and the other group received a placebo (n = 65); 90 +/- 9.2 doses were given over 120-150 d. Stools were examined for prevalence and intensity of helminths and prevalence of protozoa at the beginning and end of the study. The initial prevalence was 42% for helminths and 18% for protozoa, with no differences between groups. Mebendazole was administered to all children, and protozoal infections were treated specifically at the beginning of the study. The reinfection rates were 17% (11 of 65) for helminths and 12.3% (8 of 65) for protozoa in the zinc group and 15% (10 of 65) and 10.7% (7 of 65), respectively, in the placebo group (P > 0.05). Analysis by specific parasites revealed no treatment effect. We conclude that neither plasma or hair zinc status nor oral zinc supplementation had an effect on parasite status in children.