The primary objective of this trial is to observe the Multiple Sclerosis (MS) related fatigue during treatment with Tysabri as measured by changes in the fatigue scale for motor and cognitive functions (FMSC) over the course of 12 months. The secondary objectives are: To investigate changes in fatigue, capacity for work, Health Related Quality of Life (HRQol), sleepiness, cognitive impairment, physically activity induced exhaustion, speed of walking, status of MS disease progression and amount of walking at different times points after initiation of Tysabri treatment in participants diagnosed with Relapsing-Remitting Multiple Sclerosis (RRMS). Changes in fatigue are measured at 3, 6 and 9 months, whereas changes in capacity for work, HRQoL, sleepiness, cognitive impairment, physical activity induced exhaustion, speed of walking, status of MS disease progression and amount of walking are measured at 6 and 12 months. To investigate correlation between fatigue and cognitive impairment, depression and physically activity induced exhaustion and status of MS disease progression in participants at baseline, 6 and 12 month of treatment with Tysabri and to document any changes in fatigue related medication.