Approximately half of all elderly patients have elevated blood pressure, and proper treatment of this disorder leads to decreased cardiovascular morbidity in patients 65 and older. This study examined the effect of initial drug choice and comorbidity on medication compliance. We conducted a retrospective follow-up of 8643 outpatients aged 65 to 99 with newly prescribed antihypertensive therapy (AHT) from 1982 to 1988 in the New Jersey Medicaid and Medicare programs. Compliance was measured in terms of the number of days in which AHT was available to the patient during the 12 months following the initiation of therapy. Odds ratios (OR) and 95% confidence intervals (CI) for the outcome of good compliance (> or =80%) were calculated. In a logistic regression model, good compliance (> or =80%) was significantly associated with use of newer agents such as angiotensin converting enzyme inhibitors (OR 1.9, 95% CI 1.6 to 2.2) and calcium channel blockers (OR 1.7, 95% CI 1.5 to 2.1) as compared to thiazides, the presence of comorbid cardiac disease (OR 1.2, 95% CI 1.1 to 1.2), and multiple physician visits (OR 2.2, 95% CI 1.8 to 2.5). Good compliance was inversely associated with use of multiple pharmacies (OR 0.4, 95% CI 0.4 to 0.5) and number of medications prescribed overall (OR 0.8, 95% CI 0.7 to 0.9). Drug choice, comorbidity, and health services utilization were significantly associated with AHT compliance and represent important considerations in the management of high blood pressure. Noncompliance may be an important cause of treatment failure in elderly hypertensives.