In previous uncontrolled studies it has been reported that five days' treatment with furazolidone is effective for giardiasis. Efficacy of five v ten days of treatment was evaluated in a prospective, randomized study of 22 children with giardiasis. Eleven of 12 patients (92%) treated for ten days were cured based on clinical and parasitological response; one patient did not tolerate furazolidone and was treated with quinacrine hydrochloride. Only two of ten patients (20%) treated with a five-day regimen were cured. Of the other eight patients, five had prompt relapse of diarrhea after furazolidone treatment was stopped, diarrhea persisted in two, and diarrhea diminished but Giardia persisted in one. Six of eight patients in whom the five-day regimen failed were re-treated with furazolidone for ten days, and five patients were cured. Therapy with furazolidone for less than seven to ten days cannot be recommended for children with giardiasis.