BACKGROUND: The majority of diagnosed hypertensive patients receive drug therapy in addition to recommended lifestyle changes. One of the major barriers to the management of hypertension is the interruption or premature termination of therapy. OBJECTIVE: To estimate the prevalence of, and health care costs associated with, the discontinuation of antihypertensive drug therapy in a MediCal population. METHODS: The MediCal paid claims data were used to identify patients with at least one antihypertensive prescription. The date of this first prescription was used to partition the patient’s stream of paid claims into pre- and post-time periods of 6 and 12 months, respectively. MediCal data were available on an 11-year time span (July 1985-December 1996). Only the first episode of treatment for each patient was considered. Patients were further excluded if they were pregnant, they were in long term facilities before and at the keydate, and if they lost their MediCal eligibility during the study period. RESULTS: 30,200 patients met the eligibility criteria (mean age 60.8+19.4, 67.1% female). 20.8% started their therapy with a calcium-channel-blocker, 18.7% with a diuretic, and 18.6% with a beta-blocker. 94.2 % of patients either discontinued or interrupted their antihypertensive medications before the end of the study period. Average cost in the post-period (e.g., hospital, ambulatory care and all other costs but prescription and long term care) was $ 7945 for discontinuing vs. $ 7525 for continuing patients (Wilcoxon-rank-sums test, p=0.0354). CONCLUSIONS: Patients who interrupt their antihypertensive medications cause a higher health care expenditure than continuous patients. Further analyses are needed considering co-factors that may influence health care utilization.