OBJECTIVES:  To characterize use of once-daily emtricitabine and tenofovir disoproxil fumurate for HIV pre-exposure prophylaxis (PrEP) by examining length of therapy, gaps in therapy, and adherence using real-world pharmacy claims data. To examine factors associated with PrEP adherence, including demographic variables and use of Walgreens HIV specialized pharmacies (HIV-sp), which support adherence through personalized medication adherence counseling. METHODS:  We conducted a retrospective cohort study using Walgreens prescription pharmacy claims data. A random sample of PrEP users from calendar years 2013 to 2016 were included if they filled a PrEP prescription for >=90 days in the index year. Patients were excluded if they had evidence of combined ARV therapy during the study period. Average length of therapy and the proportion of patients with gaps in therapy were calculated. Medication adherence, measured as proportion of days covered (PDC), was calculated and analyzed by demographic variables and use of HIV-sp. RESULTS:  On average, patients used PrEP for 157 days consecutively without gaps; 55.6% of users had a >7 day gap in consecutive days of PrEP coverage during the index year. The mean PDC was .88 and 64.0% of users achieved high adherence (PDC>=90%). HIV-sp users were more likely than non-HIV-sp users to be highly adherent (OR=1.39, 95% CI: 1.27–1.52, p<.0001). Those 50 or older were 2.70 times more likely to be highly adherent than those 18-24 years old (CI: 2.34-3.14, p<.0001). Women were less likely to be highly adherent than men (OR=0.67, CI: 0.56-0.80, p<.0001) as were urban vs. rural PrEP users (OR=0.86, CI: 0.77-0.95, p=.0052). CONCLUSIONS:  PrEP adherence was significantly associated with age, gender, store type, and urban-rural MSA. These results highlight the importance of adherence counseling among specific populations and illustrate differences in adherence by service setting. Future research should investigate improved methods for calculating adherence and length of therapy for PrEP.