Six hundred sixty adults with diarrhea treated at Bamrasnaradura hospital, Bangkok, Thailand were investigated to determine the prevalence, seasonality, and severity of diarrhea associated with bacterial enteric pathogens in 1980 and 1981. Shigella were isolated from 27% and Vibrio parahaemolyticus from 19% of the patients studied. Enterotoxigenic Escherichia coli (5%), Salmonella (3%), non-01 Vibrio cholerae (3%), Campylobacter jejuni (1%), and Group F vibrio (less than 1%) were isolated from a smaller proportion of the patients. Shigella infections were most common in July-September, during the period of maximum rainfall in Thailand, while V. parahaemolyticus was isolated most frequently in September and October at the end of the rainy season. 0-1 Vibrio cholerae was isolated from 25% of 104 patients studied in the hot, dry spring of 1980, but was not isolated throughout 1981. Patients with cholera passed more watery stools, while those with Salmonella and Shigella most frequently had headaches, and those with Shigella more often had blood in their stools than those with other infections, or in whom no bacterial enteric pathogens were identified. Annual, seasonal, and, from a comparison with other reported surveys, geographical differences exist in the prevalence of bacterial enteric pathogens in adults with diarrhea in tropical developing countries.