OBJECTIVE: To compare medication adherence calculated from four different data sources including a pill count and self-report obtained during a home medication history, as well as calculations based on refill frequency derived from a provincial prescription claims database (manual and electronic). DESIGN: Baseline medication adherence was collected as part of a prospective, randomized, controlled study. Mean medication adherence results obtained from the four data sources were compared using repeated-measures ANOVA followed by a Tukey's multiple range test. SETTING: A pharmacy consultation service located at an interdisciplinary wellness center for noninstitutionalized elderly. PATIENTS: 65 years or older, noninstitutionalized, taking one or more prescribed or nonprescribed medications. Clients would either present to the wellness center or be referred by the Provincial Home Care program. RESULTS: When calculated from self-report or manual or electronic prescription claims data, mean percent adherence by drug was high and not statistically different (95.8% ± 17.1%, 107.6% ± 40.3%, and 94.6% ± 24.0%, respectively), whereas the pill count adherence was significantly lower at 74.0% ± 41.5% (p < 0.0001). CONCLUSIONS: An unexpected finding was that the pill count technique used in this study of elderly clients using chronic, repeat medications appeared to underestimate medication adherence. Numerous other limitations of pill count, self-report, and a province-wide prescription claims database in estimating medication adherence are presented. When using medication adherence as a process measure, the researcher and practitioner should be aware of the limitations unique to the data source they choose, and interpret data cautiously.