A randomised clinical trial was carried out to explore the efficacy of single dose tetracycline therapy in cholera. One hundred and eighteen adult patients were assigned to receive either tetracycline in a single 1 g, or a single 2 g dose, or tetracycline 500 mg every six hours four times, or no antibiotics as controls. The means of total liquid stool volumes after treatment were lower in the single 1 g dose group (168.0 +/- 20.9 ml/kg), in single 2 g dose group (229.5 +/- 45.6 ml/kg), and multiple dose group (214 +/- 28.5 ml/kg), than in the control group (499.1 +/- 56.5 ml/kg) (p less than 0.05). Similarly, the means of durations of diarrhoea and intravenous fluid requirements were significantly lower in the single dose and multiple dose tetracycline groups, than in the controls (p less than 0.05). The mean durations of excretion of Vibrio cholerae were significantly shortened from 3.9 +/- 0.2 days in the control group to 1.9 +/- 0.2 days in single 1 g dose, to 2.2 +/- 0.4 days in single 2 g dose and 1.3 +/- 0.1 days in multiple dose groups, respectively (p less than 0.05). Three patients in the single 1 g dose group and two patients in single 2 g dose group had clinical relapses with excretion of V cholerae during the relapses, but this was not significantly more frequent than that in the multiple dose group (p greater than 0.05). These findings suggest that although multiple dose tetracycline therapy remains the best choice, a single dose of either 1 g or 2 g tetracycline appears to be a reasonable alternative for the treatment of cholera as an adjunct to rehydration therapy.