The effectiveness and costs of care for hypertension are examined in a stratified random sample of 3,087 patients from a network of 32 Veterans Affairs Hypertension Screening and Treatment Clinics (HSTP). During 2.5 years of follow-up, 66% and 88% of patients, respectively, had mean diastolic blood pressure (DBP) levels of 90 or 95 mm Hg or less; 73% remained fully in care; and the mean cost of ambulatory care per patient-year was $647 in 1989 dollars. Higher follow-up DBP levels were found in patients who were younger, had higher DBP levels, or were receiving medication on their first visits to a clinic, were receiving more intense treatment regimens at the beginning of the follow-up period, or had been under the care of the clinic for shorter periods. Patients who were more likely to remain in care were older, received more intense treatment regimens, had prior cardiovascular complications, or had been under the care of the clinic for a longer time. Higher annual costs were associated with higher entry DBP levels, shorter durations of care, more intense regimens, and prior cardiovascular complications. Overall, patient characteristics explained 13% of the variance in mean follow-up DBP, and 31% of variance in costs. Wide variations were found among clinics in clinical outcomes and costs. After controlling for differences in patient characteristics, clinic characteristics associated with better blood pressure control were more frequent clinic visits, shorter waiting times, more time spent in patient counseling, having therapists who had a single supervisor, and better staff satisfaction.(ABSTRACT TRUNCATED AT 250 WORDS)