Poor adherence to medication is commonplace and contributes to poor health outcomes among numerous patient populations. Studies that have examined treatment adherence in multiple sclerosis focus exclusively on retrospective self-reports and/or imprecise measures of treatment discontinuation. To help address these methodological limitations, the present longitudinal study compared adherence outcomes for patients with multiple sclerosis using retrospective self-reports, adherence diaries, and a novel electronic monitoring device. Sixty-seven patients with relapsing-remitting multiple sclerosis were followed for a period of eight weeks during which they used a medication diary and a sharps container that captured electronically the time and date of each needle disposal. The patients also reported at the outset and conclusion of the study how frequently they missed doses. All measures of adherence were highly correlated. Patients reported better adherence than was indicated by medication diaries and electronic monitoring of needle disposals. Nearly one-fifth of the sample exhibited poor adherence, missing more than 20% of their prescribed medication. The results support the validity of electronic monitoring of needle disposal as an effective means of measuring adherence to disease modifying therapies in multiple sclerosis. In contrast, studies employing only self-report may underestimate poor adherence. Larger scale studies that employ prospective objective methods are necessary to gain a better understanding of adherence patterns in multiple sclerosis.