 The primary objective of the study is to determine the association between prospectively measured disability Expanded Disability Status Scale EDSS with quality of life Multiple Sclerosis International Quality of Life Questionnaire MusiQoL at 3-month intervals up to 1 year in a real life setting of multiple sclerosis MS patients treated with natalizumab. The secondary objectives of this study are as follows: To evaluate the cumulative probability of sustained EDSS changes at 1 year following natalizumab treatment initiation; To evaluate the association between disability EDSS, fatigue Fatigue Scale of Motor and Cognitive Function FSMC, sexual dysfunction Multiple Sclerosis Intimacy and Sexuality Questionnaire-19 MSISQ-19, depression Beck Depression Inventory-Fast Screen BDI-FS and neurocognitive function Symbol Digit Modalities Test SDMT with EuroQol-5D Questionnaire EQ-5D at 3 to 6-months intervals up to 1 year after initiation of natalizumab treatment; To assess the relationship between clinical disease-free status no EDSS increase of 1.0 and no relapse and MusiQoL at 1 year following natalizumab treatment initiation; To record the number of clinical relapses and relapses requiring steroid treatment at 3-months intervals up to 1 year after initiation of natalizumab treatment; To describe changes in work impairment Work Productivity and Activity Impairment in MS WPAI-MS at 3-months intervals up to 1 year after initiation of natalizumab treatment; To describe any change in the percentage of disability pension and occupation after 1 year of natalizumab treatment; To record the incidence and number of Serious Adverse Events SAE and Suspected Unexpected Serious Adverse Reactions SUSAR throughout the study 