OBJECTIVES: To examine the influence of intranasal corticosteroid (INS) adherence on 18-month outpatient use and costs among patients newly diagnosed with allergic rhinitis (AR). METHODS: This was a 12-year (6/1997-7/2009) retrospective matched cohort study in United States (U.S.) Florida Medicaid enrollees with newly diagnosed AR (≥1 year preceding initial AR diagnosis without any claim for ICD-9 477.0, 477.8, 477.9).  Selected were patients receiving ≥1 INS fill after their initial AR diagnosis with sufficient data (≥1 year of data before and ≥3 years of data after index INS fill) for analysis.  “Adequate” adherence was defined as an INS medication possession ratio (MPR; calculated as the total days of supply during 18 months) ≥70% following patient’s index INS fill, and inadequate adherence as MPR <70%.  Patients with adequate INS adherence (Adequate-INS) were matched 1:3 to those with inadequate adherence (Inadequate-INS) on age at initial AR diagnosis; sex; race/ethnicity; and Charlson Comorbidity Index and comorbid atopic illness (asthma, atopic dermatitis, and conjunctivitis) 1 year before index INS fill.  Wilcoxon signed-rank tests compared median outpatient use and costs (in U.S. dollars; USD) between Adequate-INS and Inadequate-INS groups over 18 months. RESULTS: Among all enrollees (N=7,524,231), 75,337 patients aged ≥12 years were newly diagnosed with AR.  Of these, 343 Adequate-INS patients were matched to 698 Inadequate-INS patients.  Demographics and comorbid illness rates did not significantly differ between groups at baseline.  Median 18-month number of outpatient visits (19.0 versus 25.0, p<0.0001) and costs ($1,633 versus $2,552 in USD, p<0.0001) were significantly lower among Adequate-INS versus Inadequate-INS patients.   CONCLUSIONS: Significant differences between adherence groups in number and costs of outpatient visits were observed over 18 months following INS initiation among these U.S. patients receiving State Medicaid Health care coverage.  Results suggest that INS adherence may play a role in improving health outcomes of patients with AR.