The objective was to compare the compliance of hypertensive patients treated with captopril twice daily or trandolapril once daily. After a 2-week placebo period, hypertensive patients (diastolic BP 95-115 mm Hg) were randomly allocated to trandolapril 2 mg once daily or to captopril 25 mg twice daily for 6 months. Trandolapril and captopril were packed in electronic pill-boxes equipped with a microprocessor that recorded date and time of each opening (MEMS). Patients' compliance was assessed both by standard pill-count and by electronic monitoring. Blood pressure was measured using a validated semi-automatic device at the end of the placebo period and of the treatment period. One hundred sixty-two patients entered the study. Compliance data were evaluable for 133 patients (62 in the captopril group and 71 in the trandolapril group). Treatment groups were comparable at baseline except for age (P = .046). Using electronic pill-box, overall compliance was 98.9% in the trandolapril group and 97.5% in the captopril group (P = .002). The percentage of missed doses was 2.6% in the trandolapril group and 3.3% in the captopril group (P = .06). The percentage of delayed doses was 1.8% in the trandolapril group and 11.7% in the captopril group (P = .0001). The percentage of correct dosing periods, ie, a period with only one correct recorded opening, was 94.0% in the trandolapril group and 78.1% in the captopril group (P = .0001). Results were unchanged when adjusted for age. At the end of the study, 41% of patients in the trandolapril group and 27% in the captopril group (NS) had their blood pressure normalized (systolic BP <140 and diastolic BP <90 mm Hg). In this 6-month study, the electronic pill-box allowed refined analysis of compliance of hypertensive patients. Patients' compliance with once daily trandolapril was higher than with twice daily captopril. The between-group difference is mainly explained by an increase in delayed doses in the twice daily group.