NEW YORK, Aug 1 (Praxis Press) A definite diagnosis of multiple sclerosis (MS) can only be reached after the second neurologic relapse occurs. Therefore, between the first and second relapse, patients are classified as having probable MS. To investigate the rate of progression from probable to clinically definite MS, Achiron and colleagues performed a 7-year prospective study of 163 patients experiencing their first episode of neurologic symptoms suggestive of MS. They found that within the follow-up period (mean, 42 months), 136 patients had an additional relapse and were therefore defined as having clinically definite MS, whereas 27 patients were defined as having clinically probable MS. The researchers also analyzed demographic and clinical parameters at presentation to identify variables predictive of rapid progression (within one year) to clinical definite MS. Motor involvement at onset was the only clinical parameter associated with rapid progression to a definite diagnosis. Polysymptomatic involvement and higher Extended Disability Status Scale score at presentation also correlated with rapid progression to definite diagnosis. Most patients with a diagnosis of probable MS will progress rapidly to clinically definite MS.