OBJECTIVE: To evaluate how well hypertension is managed in HMO patients and to assess opportunities for improvement. STUDY DESIGN: Retrospective cohort study. PATIENTS AND METHODS: The study population included HMO members (age 45-84 years) who had at least 1 ambulatory encounter with an ICD-9-CM diagnosis code of essential hypertension during the first 6 months of 1999. Medical records were reviewed to obtain information on blood pressure measurements, sex, age, coexisting medical conditions, smoking status, and changes made to the antihypertensive drug regimen. RESULTS: We identified 681 members with 3347 encounters related to hypertension management during 1999. Overall, 74 (11%) patients were at target blood pressure for all visits and 260 (38%) were at target blood pressure for at least 50% of the visits; 222 (33%) patients were not at target blood pressure for any visit. A history of coronary artery disease or cerebrovascular disease was associated with better blood pressure control (defined as being at goal levels during at least 50% of visits), while being older (age > or = 75) or having diabetes mellitus was associated with poorer control. Medication regimen intensifications occurred in 10% of visits with systolic blood pressure levels of 140-149 mm Hg, compared with 45% of visits with levels of > or = 180 mm Hg. Medication regimen intensifications occurred in 21% of visits with diastolic blood pressure levels of 90-99 mm Hg and 43% of visits with levels of > or = 100 mm Hg. CONCLUSION: Efforts are required to reduce "therapeutic inertia," particularly in patients with modestly elevated systolic blood pressure levels.