Cognitive dysfunction is a common symptom of Multiple Sclerosis (MS), and patients would benefit from a measure that estimates their risk of future decline. Previous work suggests that thalamic volume may have value as a predictive measure. Using 7 tesla MRI, we measured thalamic volume in 79 participants with MS. We found a strong cross-sectional relationship between volume and verbal episodic memory. The longitudinal relationship was significant, but was strongly modulated by disease duration. The current report highlights the need for careful modeling of potential confounding variables.