Abstract: ObjectiveTo explore influential factors for the evolvement from clinically isolated syndromes(CIS)into clinical definite multiple sclerosis(CDMS).MethodsTotally 123 CIS inpatients in a hospital from 2000 to 2007 were recruited and interviewed to collect information on outcome of the disease and the incidence of CDMS.Cox survival analysis was adopted in data analysis.ResultsAmong the 123 CIS patients,48 evolved into CDMS.Cox multivariate analysis showed that significant risk factors for the evolvement of CDMS among the CIS patients were with inducement before the incidente of CIS(relative riskRR=2.6,95% confidence intervalCI=1.19-5.68,P=0.02),first onset of CIS in summer(compared to the onset in winter:RR=5.48,95% CI=1.84-16.34,P=0.002),first onset of CIS in spring(compared to the onset in winter:RR=4.01,95% CI=1.16-13.89,P=0.03).ConclusionThe inducement before the incidence of CIS,first onset of CIS in summer or in winter may be risk factors for evolvement CDMS in patients with CIS.