Epidemics of cholera caused by Vibrio cholerae O1 occur regularly in Bangladesh, but until lately V cholerae non-O1 has been associated only with sporadic cases of diarrhoeal disease in many parts of the world, including Bangladesh. We describe a large epidemic of cholera-like disease in Bangladesh that is due to a V cholerae non-O1. The epidemic began in December, 1992, in southern Bangladesh and spread throughout the country. By the end of March 107,297 cases of diarrhoea and 1473 deaths had been reported. The disease is indistinguishable from cholera in clinical features and response to treatment, but most of the cases are in adults, which suggests that the population has no previous immunological experience of the organism. At two centres 375 (40%) of 938 and 236 (48%) of 492 rectal swabs were positive for V cholerae non-O1, as were 5 of 54 surface water samples. 55 isolates of V cholerae non-O1 were studied in detail. They resembled El Tor vibrios in being resistant to polymyxin B and positive for agglutination of chicken erythrocytes. The strain did not belong to any of the 138 known V cholerae serogroups; so a new serogroup O139, with the suggested name Bengal, is proposed. All the isolates studied produced large amounts of an enterotoxin apparently identical to cholera toxin. This strain seems to have pandemic potential. It is important that other countries in southeast Asia are aware of the strain's potential to cause severe morbidity and mortality.