BACKGROUND: Single-dose ciprofloxacin is effective for the treatment of severe cholera in adults. We assessed whether single-dose ciprofloxacin would be as effective as 3-day, 12-dose erythromycin in achieving clinical cure in children with severe cholera. METHODS: We did a randomised, open label, controlled trial in children age 2-15 years with V cholerae O1 or O139 present in stool on dark-field microscopy. Children received either a single 20 mg/kg dose of ciprofloxacin (n=90) or 12.5 mg/kg of erythromycin (n=90) every 6 h for 3 days, and remained in hospital for 5 days. The primary outcome was clinical success of treatment, defined as cessation of watery stools within 48 h of start of drug treatment. Analysis was per protocol. This study is registered with the ClinicalTrials.gov Protocol Registration System at http://www.clinicaltrials.gov (registration number NCT 00142272) [corrected] FINDINGS: Of 180 children randomised 162 completed the study. Treatment was clinically successful in 60% (47/78) of children treated with ciprofloxacin and in 55% (46/84) of those treated with erythromycin (difference 5% [95% CI -10 to 21]). Children receiving ciprofloxacin vomited less often (58%vs 74%; difference 16% [2 to 30]), had fewer stools (15 vs 21; 6 [0 to 9]), and less stool volume (152 vs 196 mL/kg; 43 mL/kg [13 to 87]) than those receiving erythromycin. Bacteriological failure was more common in ciprofloxacin-treated patients (58%vs 30%; 28% [13 to 43]) than erythromycin-treated patients. INTERPRETATION: Single-dose ciprofloxacin achieves clinical outcomes similar to, or better than, those achieved with 12-dose erythromycin treatment in childhood cholera, but is less effective in eradicating V cholerae from stool.