Background: A diagnosis of multiple sclerosis in patients who present for the first time with a clinically isolated syndrome (CIS) can be established with brain magnetic resonance imaging (MRI) if the MRI demonstrates demyelinating lesions with dissemination in space (DIS) and dissemination in time (DIT). Objective: To investigate the diagnostic performance of a single MRI study obtained within the first 3 months after symptom onset in a cohort of patients with a CIS suggestive of multiple sclerosis at presentation. Design: Multicenter inception cohort with a follow-up of at least 24 months. Setting: Referral hospitals. Patients: Patients with CIS onset between April 1, 1995, and September 30, 2004, who fulfilled the following criteria were included: (1) age of 14 to 50 years and (2) clinical follow-up for at least 24 months after CIS onset or until development of clinically definite multiple sclerosis (CDMS), if this occurred within 2 years. Main Outcome Measure: All patients underwent 2 comparable brain MRI examinations, the first within 3 months (early) and the second between 3 and 12 months (delayed) after CIS onset. We defined DIS using several existing MRI criteria, and DIT was inferred when there were simultaneous gadolinium-enhancing and nonenhancing lesions on a single MRI. Results: Two hundred fifty patients were included in the study. The comparison of the diagnostic performance of various MRI criteria for identifying early converters to CDMS showed similar sensitivity and specificity between early and delayed MRIs. In addition, the use of less stringent criteria for DIS yielded better sensitivity and similar specificity, particularly when assessed in the first weeks after CIS onset. Conclusion: A single brain MRI study that demonstrates DIS and shows both gadolinium-enhancing and nonenhancing lesions that suggest DIT is highly specific for predicting the early development of CDMS, even when the MRI is performed within the first 3 months after the onset of a CIS. ©2009 American Medical Association. All rights reserved.