OBJECTIVES: to determine the prevalence and characteristics of previously unknown methicillin-resistant Staphylococcus aureus (MRSA) carriers at admission. DESIGN: two prospective case-control studies. SUBJECTS: 1,621 elderly patients were screened for MRSA carriage within 24 hours after admission to a geriatric hospital in Geneva, Switzerland. METHODS: risk factors associated with previously unknown MRSA carriage were determined in the derivation group, and the resulting risk score was evaluated in the validation cohort using logistic regression analysis. RESULTS: prevalence of MRSA carriage at admission increased from 7.3% (53/724 patients) in 2001 to 8.7% (78/897 patients) in 2003, with a corresponding prevalence of unknown MRSA carriers of 4.6 and 5.8%, respectively. Three variables were independently associated with previously unknown MRSA carriage: recent antibiotic treatment (adjusted OR (aOR) 2.3; 95% CI 1.0-5.1), intra-hospital transfer (aOR 2.5; 95% CI 1.2-5.3), and hospitalization in the past 2 years (aOR 2.7; 95% CI 1.1-6.7). In the validation cohort, the probability of MRSA carriage increased across risk scores: 0 point, 4% prevalence (6/146); 1 point, 15% (21/136); and