In June 1997, an outbreak of enterohemorrhagic Escherichia coli O157 occurred at a hospital of Okayama City, Japan. E. coli O157 was isolated from 86 patients (40 males and 46 females). Ages ranged from 14 to 96 years old with a mean of 53 years old. All the infected patients (59 asymptomatic carriers) were investigated in this study. The median duration of shedding (from starting therapy), among the 83 patients who received antimicrobial therapy, was 6 days. This result has paticular importance for taking appropriate measures during outbreaks among adults who have other diseases. Other factors (age,sex,etc) that might have affected the duration of shedding were aiso investigated, but were not found to be influential. Among the infected patients, elderly people, females and patients who had other diseases became symptomatic, and in paticular, patients who had severe malignancy became symptomatic. All asymptomatic carriers received antimicrobial therapy and no newly affected cases and no side effects among those patients were observed. these results indicate that antimicrobial therapy for asymptomatic carriers in facilities that have a large number of susceptible people is useful.