Between July and October 1987 an outpatient study of 191 children with acute diarrhea was undertaken in two rural communities in Mexico. Through a double-blind randomization we compared the efficacy of a combination therapy of furazolidone, 7.5 mg/kg/day, plus standard oral rehydration therapy (ORT) (96 patients) versus a placebo plus ORT (95 patients), each given for 5 days. Diarrheal stool samples were collected from all patients before therapy. By means of a two-vial transport media system the samples were sent to a university laboratory and examined for viral, bacterial, and parasitic organisms. The most commonly isolated organisms were enterotoxigenic Escherichia coli (13%) and Giardia lamblia (13%). Patients who received furazolidone plus ORT showed a greater reduction in duration of diarrhea when compared with those receiving placebo plus ORT (63.4 h versus 71.44 h). There was also a trend toward shorter duration of diarrhea in patients with Giardia who were treated with furazolidone/ORT compared with Giardia patients in the placebo/ORT group. When fecal leukocytes were present in the stool, the furazolidone/ORT-treated patients had a significantly higher percentage of clinical cures (79% versus 54%, p = 0.03) and an overall shorter duration of diarrhea (62.0 h versus 80.6 h, p = 0.055) at the end of 5 days of therapy than did the placebo/ORT-treated group.