Rectal swabs obtained from 10 of 49 chronic-care geriatric patients were positive for Clostridium difficile, for a prevalence rate of 20.4%. Simultaneous sampling of 29 patients in an acute geriatric ward revealed four colonized patients, for a prevalence rate of 13.7%. A prospective study of C. difficile colonization in 100 consecutive patients admitted to an acute geriatric ward was carried out. All patients were sampled upon admission and biweekly during hospitalization. Two patients (2%) were positive on admission, and 12 of the 98 initially negative patients became colonized, representing a nosocomial acquisition rate of 12.2%. A major determinant for C. difficile colonization in asymptomatic patients appears to be length of hospitalization. Previous antibiotic administration was not found to be a significant factor.