['Based on the theories of brain reserve and cognitive reserve, we investigated whether larger maximal lifetime brain growth (MLBG) and/or greater lifetime intellectual enrichment protect against cognitive decline over time.', 'Forty patients with multiple sclerosis (MS) underwent baseline and 4.5-year follow-up evaluations of cognitive efficiency (Symbol Digit Modalities Test, Paced Auditory Serial Addition Task) and memory (Selective Reminding Test, Spatial Recall Test). Baseline and follow-up MRIs quantified disease progression: percentage brain volume change (cerebral atrophy), percentage change in T2 lesion volume. MLBG (brain reserve) was estimated with intracranial volume; intellectual enrichment (cognitive reserve) was estimated with vocabulary. We performed repeated-measures analyses of covariance to investigate whether larger MLBG and/or greater intellectual enrichment moderate/attenuate cognitive decline over time, controlling for disease progression.', 'Patients with MS declined in cognitive efficiency and memory (p < 0.001). MLBG moderated decline in cognitive efficiency (p = 0.031, ηp (2) = 0.122), with larger MLBG protecting against decline. MLBG did not moderate memory decline (p = 0.234, ηp (2) = 0.039). Intellectual enrichment moderated decline in cognitive efficiency (p = 0.031, ηp (2) = 0.126) and memory (p = 0.037, ηp (2) = 0.115), with greater intellectual enrichment protecting against decline. MS disease progression was more negatively associated with change in cognitive efficiency and memory among patients with lower vs higher MLBG and intellectual enrichment.'] Do brain reserve and cognitive reserve protect against cognitive decline over 4.5 years in MS?