OBJECTIVES: To determine the prevalence and epidemiological characteristics of rotavirus among adults admitted to the hospital with diarrhea that have bacterial stool cultures sent. METHODS: The prevalence of rotavirus was determined by Rotaclone EIA in samples submitted for bacterial stool culture from adults requiring hospitalization at Northwestern Memorial Hospital, Chicago from December 01, 2005-November 30, 2006. RESULTS: Rotavirus was detected in 2.9% of eligible bacterial stool cultures. A bacterial pathogen (e.g., Salmonella, Shigella, Campylobacter) was identified in 3.3%. Bacterial stool pathogens were more common from June-October while rotavirus was 2.4 times more common than all bacterial pathogens from February-May. Adults in whom rotavirus was detected were older (p < 0.05) and more often immunosuppressed (p < 0.02), particularly with HIV (p < 0.04) compared to individuals from whom bacteria were isolated. The duration of hospitalization and the number of invasive procedures performed in those with rotavirus and bacterial diarrhea were comparable. CONCLUSIONS: In the era immediately prior to widespread rotavirus vaccination of children, rotavirus was as commonly detected from adults admitted to the hospital with diarrhea as are the bacterial gastroenteritis pathogens. Rotavirus is particularly prevalent from February-May (as in children) and in immunosuppressed or older adults.