Despite rapid urbanization and increasing affluence in Karachi, cases of cholera are frequent. We analysed computerized isolation data from the AKUH Clinical Microbiology Laboratory, Karachi, from 1990–6 to examine microbiological, temporal and demographic trends in Vibrio cholerae infections. During this period 888 strains of V. cholerae (566 V. cholerae serogroup O1, and 204 V. cholerae serogroup O139) were isolated from specimens from 886 patients; 214/464 were adult inpatients, and 250/464 paediatric inpatients, the remaining 422 outpatients. Isolations peaked between June and August. Overlapping epidemics occurred in 1993 and 1994 of serogroup O1 (May to August), and serogroup O139 (August to October). All ages and social and economic strata were affected. Forty-four percent of all isolates were from children under the age of 5 years. The mean age of all patients with serogroup O1 infections was 19·6 years (±0·9) compared with 36·7 (±1·7) for serogroup O139 infections (P<0·0001, t test). More than a quarter (27%) of all serogroup O1 isolates were from babies under 2 years of age. One patient had a serogroup O1 infection followed by a serogroup O139 infection 1 year later. Another patient was infected with serogroup O1 strains 5 years apart. Emergence of resistant strains was observed, but by 1996 serogroup O139 had disappeared. An aquatic organism, cholera nevertheless continues to take its toll in this city of 11 million situated in a desert.