OBJECTIVES: To study the prevalence of comorbid conditions in a population of patients with Multiple Sclerosis (MS). METHODS: In a retrospective analysis, integrated medical and pharmacy claims data (IMS LifeLink™ Health Plan Claims and Longitudinal Prescriptions databases) were analyzed to select patients with an active diagnosis of MS (ICD-9 code 340.*) during the 2009 calendar year. The presence of comorbidities was also determined using ICD-9 codes present on medical claims. Prescription drug use was evaluated by pharmacy claims and drug-specific billing codes. RESULTS: From the database, 18,013 patients with MS were identified. About two-thirds (67.6%) of patients were between 18 and 55 years of age, and 77.6% were female. Evidence of specific chronic conditions was substantial and included: hyperlipidemia (seen in 36.1% of patients), HTN (32.9%), diabetes (10.8%), asthma (8.3%), cardiac arrhythmias (7.2%), COPD (4.5%), and CHF (1.6%). The presence of any respiratory conditions (asthma or COPD) was present in 11.4% of patients, cardiovascular conditions (HTN, CHF, arrhythmias) in 37.3%, and metabolic conditions (DM, HTN, hyperlipidemia) in 50.5%. Though the prevalence of many conditions was age and gender dependent, their prevalence was independent of treatments including disease-modifying treatments (DMTs) such as interferon, glatiramer acetate, and natalizumab. CONCLUSIONS: This analysis illustrates the complexity of the case-mix of patients with MS. An increasing amount of evidence suggests that physical and mental comorbidities, and adverse health factors are common and can affect the disease.  Because of the high prevalence of comorbidities, any treatment decisions should be carefully considered and should be individualized in order to optimize outcomes and reduce the potential interactions between treatment and comorbidities.