OBJECTIVES: The objective of this analysis was to compare two commonly used adherence calculations for multiple sclerosis patients prescribed disease modifying drugs (DMDs) in a national managed care population.  METHODS: Patients with pharmacy claims for self-injectable DMDs were selected from 2007-2008 Thomson MedStat data.  Adherence was calculated across all DMDs for 12 months after the first DMD claim (i.e., index date) using two different adherence calculation methods.  Traditional medication possession ratio (TMPR) was calculated by summing the days supply from the first to the last prescription and dividing by the time between the last prescription date plus the days supply on the last prescription and the first prescription date.   The MMPR used the same numerator but divided by the 365 days of the follow-up period. The TMPR was calculated based on available data (no eligibility requirements) while MMPR requires continuous eligibility for the 12-month follow-up period.  RESULTS: The mean adherence value for TMPR (n=3,405) was 90.5% whereas the mean adherence value for MMPR (n=2,145) was 78.0%.   Based on TMPR, 82.3% of patients were considered adherent (≥80%) while this value decreased to 63.7% for MMPR.  CONCLUSIONS: The MPR is often used to describe patient adherence.  This adherence measure, however, can be calculated using different time periods in the denominator.  These results demonstrate the importance of understanding the adherence calculation method and the population in which the measure is generated, and the potential implications to patient care.