Background Colonization with methicillin-resistant Staphylococcus aureus (MRSA) is a risk factor for subsequent invasive MRSA infection,particularly in patients admitted for critical care.The purpose of this study was to investigate the risk factors affecting nasal colonization of MRSA in patients admitted to intensive care units (ICU).Methods Between August 1,2011 and June 30,2012,we screened for MRSA nasal colonization in 350 patients by Real-time PCR within 24 hours of admission by means of swab samples taken from the anterior nares.According to the results of PCR,the patients were divided into 2 groups:the positive group with nasal MRSA colonization and the negative group without nasal MRSA colonization.The 31 (8.86％) patients were MRSA positive.The risk factors evaluated included thirteen variables,which were analyzed by t test for continuous variables and X2 test for discrete variables.The variables with significance (P ＜0.05) were analyzed with stepwise Logistic regression.Results There were differences (P ＜0.05) in four variables between two groups.The duration of stay in hospital prior to ICU admission in the positive group was (35.7±16.1) days,vs.(4.5±3.1) days in the negative group.The average blood albumin level was (28.4±2.9) g/L in the positive group,vs.(30.5±4.3) g/L in the negative group.Of 31 patients in the positive group,seven had been treated with antibiotics longer than seven days vs.34 of 319 patients in the negative group.In the positive group,four of 31 patients received treatment with more than two classes of antibiotics prior to admission in ICU,contrasted to 13 of 319 patients in the negative group.Furthermore,stepwise Logistic regression analysis for these four variables indicates that the duration of stay in hospital prior to ICU admission may be an independent risk factor.Conclusions MRSA colonization in ICU admission may be related to many factors.The duration of stay in hospital prior to ICU admission is an independent risk factor.