Background: The MSSS, combining the EDSS and disease duration, emerged a decade ago as a way of stratifying MS patients based on their rate of progression. While useful to describe MS populations in general, its prognostic ability in the individual patient remains unproven. Objectives: To assess the stability of MSSS within individual patients with MS in a longitudinal cohort, to evaluate whether certain factors influence MSSS variability, and to explore the ability of MSSS to predict future ambulatory function. Methods: Single center retrospective review of patients following with a single provider for at least 8 years. Mixed model regression was used to model MSSS over time. A Kaplan-Meier survival plot was fitted, using change of baseline MSSS by at least one decile as the event. Cox modeling was used to determine influence of baseline clinical and demographic factors on the hazard of changing MSSS by at least one decile. Univariate and multivariate linear models evaluated the impact of baseline EDSS, baseline MSSS, and other factors on an unrelated outcome measure, the Timed 25-Foot Walk (T25FW). Results: A majority of patients (55.7%) did not stay within 1 decile of baseline MSSS. Every year increase in baseline disease duration made it 4% less likely that a patient's MSSS would change by a decile or more. Baseline MSSS was not better correlated with final T25FW than baseline EDSS, which had a moderately strong correlation with future log T25FW. Conclusion: Individual MSSS scores often vary over time. Clinicians should exercise caution when using MSSS to prognosticate in MS.