OBJECTIVES:  Identify the frequency, severity, and cost of relapses among a multiple sclerosis (MS) population. METHODS:  Data was assessed from MarketScan® Research Database.  Patients at least 18 years old with MS, defined at least 2 outpatient diagnostic codes for MS (ICD-9-CM 340) or one inpatient code during the index period (1/1/2004-6/30/2008) were included. Continuous enrollment required 6 mos pre-index and 12 mos post-index. A sub-group of newly treated patients, defined as MS patients free of DMT claims for 6 mos prior to initial MS diagnosis, were also analyzed for frequencies of relapses. Using a claims-based definition, severe relapses were defined as an inpatient hospitalization with a primary diagnosis of MS. Moderate relapses were defined as an outpatient visit with a diagnosis of MS in combination with a pharmacy or medical claim for a corticosteroid within 7 days following the outpatient visit.  Additional data collected included relapse-related costs and length of inpatient stay. All variables were analyzed descriptively.   RESULTS:  Among the 23,503 MS patients identified, mean age was 47 years and 18,027 (77%) were female.  Overall, 1,479 (6.3%) patients had a relapse.  940 patients (4.0%) had a severe relapse, with an average length of inpatient stay of 5.6 days.  Among, the sub group of newly treated MS patients (NTMS) (N= 15,059), 1,059 (7.0%) were classified as having a relapse, 710 (4.7%) were severe.  Mean cost of relapse [NTMS] was $12,558 [$11,485] for severe, and $1,561 [$1,844] for moderate. CONCLUSIONS: This descriptive analysis provides an updated estimate of the frequency and direct medical costs of both severe and moderate relapses among multiple sclerosis patients.  The ranges of health care services used to manage a relapse reflect severity level. Further database analyses evaluating the impact of disease modifying treatment on the rates and costs of relapses is warranted.