bAsymptomatic colonization may contribute to Clostridium difficile transmission. Few data identify which patients are at risk for colonization. We performed a prospective cohort study of C. difficile colonization and risk factors for C. difficile acquisition and loss in hospitalized patients. Patients admitted to medical or surgical wards at a tertiary care hospital were enrolled; interviews and chart review were performed to determine patient demographics, C. difficile infection (CDI) history, medications, and health care exposures. Stool samples/rectal swabs were collected at enrollment and discharge; stool samples from clinical laboratory tests were also included. Samples were cultured for C. difficile, and the isolates were tested for toxins A and B and ribotyped. Chi-square tests and univariate logistic regression were used for the analyses. Two hundred thirty-five patients were enrolled. Of the patients, 21% were colonized with C. difficile (toxigenic and nontoxigenic) at admission and 24% at discharge. Ribotype 027 accounted for 6% of the strains at admission and 12% at discharge. Of the patients colonized at admission, 78% were also colonized at discharge. Cephalosporin use was associated with C. difficile acquisition (47% of patients who acquired C. difficile versus 25% of patients who did not; P ؍ 0.03). -lactam--lactamase inhibitor combinations were associated with a loss of C. difficile colonization (36% of patients who lost C. difficile colonization versus 8% of patients colonized at both admission and discharge; P ؍ 0.04), as was metronidazole (27% versus 3%; P ؍ 0.03). Antibiotic use affects the epidemiology of asymptomatic C. difficile colonization, including acquisition and loss, and it requires additional study.C lostridium difficile infection (CDI) is the most common infectious cause of hospital-associated diarrhea, and while most CDI infections are mild, severe CDI can lead to outcomes, such as toxic megacolon, colectomy, and death (1-4). Current estimates of the excess health care costs associated with CDI in the United States are $4.8 billion per year (5). The emergence of the epidemic NAP1/BI/027 strain of C. difficile (6-8) and increased infection rates (9, 10) have generated a renewed interest in this pathogen.Despite this interest in CDI, optimal methods for preventing CDI remain poorly understood. Traditionally, symptomatic CDI patients have been considered the primary reservoir for C. difficile transmission because they shed more C. difficile in their stool than asymptomatically colonized patients (11-13). However, recently published studies indicate only one-third or fewer of new CDI cases in the hospital setting can be attributed to transmission from another CDI case (14-17). It is possible that asymptomatic carriers are an important source of C. difficile transmission (18, 19). Curry et al. (20) found that the percentage of CDI cases related to symptomatic CDI patients and that related to asymptomatic C. difficile carriers was almost identical (30% versus 29%, respectively...