Despite the widespread availability of evidence-based guidelines for treating hypertension, recent evidence suggests that physicians may not be prescribing first-line drugs for their patients with high blood pressure. Using administrative claims data from 1998 through 2000, this study investigates whether drug treatment provided to 6,736 hypertensives in a privately insured, non-HMO population follows practice guidelines. The authors also examine physician and patient-related factors associated with guideline adherence in a subset of patients with newly diagnosed hypertension. Among members with high blood pressure alone, only 38 percent were on a diuretic, while less than a third were prescribed a beta-blocker, the JNC VI recommended first-line antihypertensives for essential hypertension. Approximately half of individuals with high blood pressure and certain comorbidities received non-first-line interventions. Such findings indicate the need to reconsider how guidelines are communicated and shared with medical practitioners and patients, particularly in light of the drug industry's promotion of newer, more expensive drugs.