The feasibility and reliability of home blood pressure measurement in clinical research were determined as part of a study to document the safety and efficacy of two captopril dosage regimens. Twenty patients were studied. Patients were instructed to record blood pressures twice daily (before morning and evening doses of captopril) for 24 weeks. All patients were instructed in blood pressure measurement and were required to demonstrate the accuracy of their techniques. Supine and standing blood pressures were measured every two weeks in the clinic. The mean of all home blood pressures recorded in the two-week period before each clinic visit was compared with the measurement obtained at that clinic visit for each patient. Blood pressure data could be evaluated for 17 patients. Each patient recorded 330-380 readings, and the mean home blood pressure measurement for each two-week period was calculated from 26-32 separate readings. Home blood pressures were consistently lower than clinic measurements but showed similar trends in response to treatment. There was good correlation between values obtained at home and in the clinic. Morning and evening blood pressures obtained by patients were not significantly different. Home blood pressure monitoring is a useful tool in hypertension research.