Prescriptions that are ordered by physicians but not picked up by patients represent a potential quality improvement opportunity in health systems. Previous research has demonstrated that anywhere from as little as 0.28% to as much as 30.0% of prescriptions are unclaimed, and that 0.45% to 22.0% of patients fail to claim prescriptions. In the Military Health System (MHS), prescriptions filled at military pharmacies are dispensed with no copayment, providing an opportunity to examine the factors that contribute to unclaimed prescriptions other than out-of-pocket cost. To estimate the prevalence of unclaimed prescriptions in the MHS, investigate reasons for unclaimed prescriptions, and compare self-reported noncompliance, defined as the failure to pick up at least 1 prescription in a 12-month period, with evidence from an administrative database of prescription orders and dispensings. Research methods included pharmacy staff interviews at 6 military pharmacies, a telephone survey of beneficiaries who filled prescriptions at these pharmacies, descriptive analysis of survey data, and comparison of administrative pharmacy data with self-reported survey data. Beneficiary interviews, conducted from May through July 2004, covered background characteristics, medical conditions, and unclaimed prescriptions, relying on 12 months of recall regarding noncompliance. Interviews with pharmacy staff covered day-to-day operations, factors that alleviate or exacerbate noncompliance, and the burden that noncompliance places on pharmacies. Administrative data from the Pharmacy Data Transaction Service (pharmacy claims) and Composite Health Care System (CHCS: prescription orders and dispensings) databases were used to select a random sample for the beneficiary survey. Survey respondents' CHCS data were matched to their responses to determine the degree of agreement between self-reports and administrative data. Pharmacy interviews were completed with 30 staff members at 6 military pharmacies, and telephone interviews were completed with 1,214 beneficiaries (60.6% response rate). Beneficiary respondents filled an average of 7 prescriptions in the 5 months approximately surrounding the survey administration time frame (from March to July 2004). More than half (56.8%) of respondents were female, and nearly 60.6% were retired military or their dependents. Among all respondents at all study pharmacies, 8.0% reported failing to claim at least 1 prescription during the prior 12 months. Among survey respondents deemed compliant by CHCS data, 93.8% correctly identified themselves as compliant. However, among patients identified as noncompliant using CHCS data, only 16.0% selfidentified as noncompliant. The administrative data were not concordant with self-report data: of 105 survey respondents identifying themselves as noncompliant in the prior year and matched to administrative data (CHCS), only 58.1% were noncompliant per administrative data, and of 1,065 selfidentifying as compliant, only 61.1% were compliant per administrative data. The most common reasons cited by respondents for not picking up their prescriptions were: no perceived need for the prescription (18.5% of the noncompliant), forgot to pick it up (17.3%), the prescription was not in stock (14.8%), long wait time (11.1%), the prescription was not yet available (10.5%), was out of town (9.9%), and was too busy to pick up the prescription (6.2%). Factors associated with unclaimed prescriptions were: younger age, active duty military status, lower educational levels, and the absence of certain chronic medical conditions (i.e., no claims for cardiovascular medications, no self-reported arthritis). The present study's survey findings of an 8.0% selfreported noncompliance rate fall in the midrange of noncompliance rates reported in previous literature: between 0.45% and 22.0% in nonmilitary populations. Although reported reasons for noncompliance were generally consistent with those identified in previously published studies, they were only partially consistent with previous military pharmacy literature, which also found that patients did not know they had a prescription waiting or had some of the prescribed medicine at home. Concordance between measures of noncompliance, comparing administrative data with patient self-report based on 12-month recall, was poor.