To evaluate single doses of 400 mg of furazolidone and 1 g of tetracycline given orally to patients with diarrhea due to Vibrio cholerae, we studied 87 adults in a randomized, double-blind, placebo-controlled trial. All patients received intravenous fluids for rehydration and no other drugs. The total volumes of stool (mean +/- standard deviation) during a 6-day period after treatment were significantly smaller in the tetracycline group (10.5 +/- 8.6 liters) than in the furazolidone group (20.9 +/- 15.9 liters) and the placebo group (19.1 +/- 10.5 liters) (P less than 0.01). The duration of diarrhea and volumes of intravenous fluids were also significantly reduced in the tetracycline group (P less than 0.05). However, there were no differences between the furazolidone and the placebo groups with regard to stool volume, intravenous fluid, and duration of diarrhea. Within 48 h of treatment, tetracycline significantly reduced the number of patients with positive stool cultures for V. cholerae (37%) compared with furazolidone treatment (96%) and the placebo (97%) (P less than 0.001). Although the tetracycline group had a significantly higher incidence (61%) of bacteriologic relapse (negative stool cultures on days 2 and 3, followed by positive cultures afterward) compared with that in the furazolidone group (40%) and the placebo group (33%), this was not associated with clinical relapse. There were no differences between the furazolidone and placebo groups with regard to any of the bacteriologic responses examined. These data indicate that a single dose of 1 g of tetracycline is effective in the treatment of cholera, but it is asymptomatic bacteriologic relapse. A single dose of 400 mg of furazolidone is not therapeutically effective in cholera.