OBJECTIVES: Despite numerous studies on medication adherence, there is little research on the first prescription fill rate.  Our study used claims data from electronic prescribers to calculate first fill failure rate for antihypertensive prescriptions. METHODS: This retrospective study combined administrative, medical and pharmacy claims data from a health plan to find the percentage of unclaimed, first fill antihypertensive prescriptions, the primary outcome.  Adult members with new antihypertensive prescriptions prescribed by an electronically prescribing physician were eligible for inclusion.  We allowed only prescriptions written by physicians who were electronically prescribing in order to capture antihypertensive prescriptions that would likely be adjudicated with a claim captured.  Each antihypertensive prescription was assigned one outcome:  paid claim (patient obtained prescription) and denied claim (unclaimed prescription).   RESULTS: The cohort consisted of 14,693 new antihypertensive prescriptions prescribed by 164 electronically prescribing physicians.  First fill failure rate was 15.6% for prescriptions which affected 24.3% of patients.  We examined the accuracy of this new methodology by verifying the prescription rate through medical record abstraction.   Chart review occurred for 183 of the 200 charts.  There were 254 antihypertensives prescribed, of which 247 (97.2%) had a matching pharmacy claim.  Our methodology identified 97.2% of the prescribed antihypertensives and implies that 2.8% of prescriptions would not have been identified.  CONCLUSIONS: Using electronic prescribers to proxy electronic prescribing is a new method for assessing first fill rates.  The intent was to identify most of the prescribed antihypertensives so that matching pharmacy claims would determine whether the prescription was obtained.  With a potential error rate of 2.8% of prescriptions, this methodology appears sensitive and accurate to determine adherence to first fill prescriptions.  Future research should examine the correlation between the use of electronic prescriber data and electronic prescriptions.