A two-phase study was conducted to assess the effect of an electronic medication compliance aid on hypertension control and pharmaceutical compliance in ambulatory patients. In Phase I (12 weeks), 36 patients were randomly assigned to a medication vial equipped with a cap containing a digital timepiece that displays the last time the cap was removed. The control group included 34 patients randomly assigned to a standard medication vial. Subjects using the timepiece cap showed an average compliance rate of 95.1%, an average decrease in systolic pressure of 7.6 mm Hg (P = .006), and an average decrease in diastolic pressure of 8.8 mm Hg (P less than .001). Controls had an average compliance rate of 78% and decreases of 2.8 mm Hg and 0.2 mm Hg in systolic and diastolic pressures, respectively. Phase II (12 weeks) combined use of the timepiece cap with other compliance aids: a pocket-size card for recording blood pressure and a blood pressure cuff for self-monitoring. Patients using the timepiece cap and the card had an average compliance rate of 98.7% with mean decreases of 11 mm Hg in systolic pressure (P less than .01) and 7.64 Hg mm in diastolic pressure (P = .0001). The combined use of the cap, the card, and the blood pressure cuff resulted in an average 100.2% compliance rate with mean decreases of 15 mm Hg (P = .0006) and 6.60 mm Hg (P = .0006) in systolic and diastolic pressures, respectively. Results of the two-phase study showed statistically significant increases in medication compliance associated with statistically and clinically significant reductions in blood pressure for all patients using the timepiece cap.