BACKGROUND The role of Clostridium difficile (CD) carriers in health care-associated CD transmission has been identified as an area needing research. We investigated the prevalence of, and risk factors for, asymptomatic CD colonization at hospital admission. METHODS Adults admitted to a tertiary care hospital in Minnesota on predetermined study days between March 1 and April 30, 2009, and without symptoms of C difficile infection, were eligible. The first stool sample after admission was requested from each consenting patient and tested for toxigenic CD using polymerase chain reaction (PCR) that detects tcdC. Clinical data were obtained through interviews and chart reviews. RESULTS Of 320 participants, 31 (9.7%) were positive for toxigenic CD. Using multivariate logistic regression, independent predictors of CD colonization were recent hospitalization (odds ratio [OR], 2.45; 95% confidence interval [CI]: 1.02-5.84), chronic dialysis (OR, 8.12; 95% CI: 1.80-36.65), and corticosteroid use (OR, 3.09; 95% CI: 1.24-7.73). Screening patients with risk factors (48% participants) would identify 74% (95% CI: 55%-88%) of CD carriers. CONCLUSIONS Asymptomatic CD colonization at hospital admission was detected in nearly 1 of 10 patients. The majority of colonized patients had one or more identifiable risk factors. These data could provide the basis for designing studies of targeted surveillance for C difficile.