Purpose: The purpose of this study was to describe the Medication Management Instrument for Deficiencies in the Elderly (MedMaIDE) and to provide results of reliability and validity testing. Design and Methods: Participants were 50 older adults, aged 65 and older, who lived in the community, took at least one prescription medication, and were then self-medicating. Nonmedical study staff assessed participants in their homes at baseline and 1 week, and a study pharmacist conducted pill counts at baseline and 30 days. The MedMaIDE covers three domains important for ensuring medication compliance (knowledge of medications, how to take medications, and procurement) and yields a total deficiency score. We assessed test-retest and interrater reliability. We assessed validity by comparing the MedMaIDE deficiency scores to 30-day pill count compliance. Results: The sample was mostly female (72%) and White (56%), with a mean age of 78. Participants were taking an average of 7 prescription drugs, with an average pill count compliance of 70%. The MedMaIDE had very good test-retest reliability (intraclass correlation coefficient [ICC] = 0.93) and good interrater reliability (ICC = 0.74). Internal consistency was also strong (Cronbach's alpha = 0.71). Comparing the MedMaIDE to the pill count with those who were compliant (greater than 80%) versus those that were not, the agreement was 75%. The MedMaIDE was more highly specific and predictive of compliance compared to the pill count. Implications: The MedMaIDE appears to be a reliable and valid instrument for determining if an older adult has deficiencies in managing medications.