Results of a prospective, open and non-comparative study of efficacy of lomafloxacin 400 mg once a day for 3 days in adults with severe cholera are presented. Seventy-three patients were enrolled; 68 of them were evaluable for clinical efficacy, all of them had Vibrio colerae El Tor isolated from their stool cultures. Ninety-one percent (62 patients) had shock on admission. The mean volume of stools during the first four hours of rapid intravenous rehydration was 2.227 ± 1.328 ml. The mean duration of diarrhea was 49.6 ± 16.9 hours (range 24-99 hours). The clinical cure was 61/68 (90%), and the 7 patients with clinical failure had a reduction of 85% in the diarrhea volume. The microbiological cure was 100% at the end of treatment, including 7 patients with clinical failure. At the first day of treatment 52% of stool cultures were negative, and after 48 hours 87% were negative. All the strains isolated were susceptible to lomefloxacin, MIC90 = 0.06 μg/ml. Seven patients had adverse effects: 5 had vomiting; 1 had transitory urinary retention caused by a prostatic adenoma; and 1 patient had insomnia.