OBJECTIVES: Patients with multiple sclerosis (MS) who have relapses are treated with corticosteroids and sometimes admitted to the hospital. The objective of this study is to examine changes in corticosteroid utilization, MS-related inpatient hospitalization, and related costs in MS patients before and after initiating natalizumab treatment in a real-world setting. METHODS: A retrospective administrative claims analysis was conducted using a large US commercial and Medicare supplemental database. The study population included adult patients diagnosed with MS who initiated natalizumab treatment between January 1, 2007, and December 31, 2010 (index), had not received any MS disease-modifying therapy for 12 months prior to index, and had ≥24 months of continuous data (12 months before and after index). Patient characteristics at index and corticosteroid use, MS-related inpatient hospitalizations, and related costs 12 months before (pretreatment) and 12 months after (posttreatment) natalizumab treatment initiation were analyzed and described using paired statistical tests. RESULTS: Data were from 535 patients: 70.1% female, mean age 45.8 (standard deviation 11.2) years. Compared with the previous 12 months, the proportions of patients with oral and IV corticosteroid use significantly decreased by 10.3% (28.6% pretreatment vs 18.3% posttreatment; P<0.001) and 16.7% (34.8% pretreatment vs. 18.1% posttreatment; P<0.001), respectively, after 12 months from natalizumab initiation. Mean expenditure per patient of oral and IV corticosteroid prescriptions decreased by 57.1% (P=0.002) and 54.7% (P=0.007), respectively. A significant reduction was observed in the proportion of patients with MS-related inpatient hospitalizations (7.3% pretreatment vs. 3.0% posttreatment; P=0.001), as well as in MS-related inpatient hospital expenditures among patients with an inpatient admission (median cost per patient: $12,078 pretreatment vs $9,289 posttreatment; P=0.001). CONCLUSIONS: Indicators of MS relapses, MS-related inpatient hospitalizations and corticosteroid use, were significantly reduced in MS patients 12 months after initiating natalizumab treatment in a real-world setting both in terms of costs and health resource utilization.