The study of pharmacist counseling on ambulatory hypertensive patients at Chaiya hospital was done during November 1998 to April 1999 by followed up and counseling for 6 months consecutively. The objective of the study was to see the effect of pharmacist counseling on the patients by comparing the results before and after the service was implemented: the number of patients with the change in blood presssure, the knowledge about hypertension and treatment, noncompliance, adverse drug reactions and drug interactions that occurred and the time and the cost spent on counseling by pharmacist. The ambulatory hypertensive patients already on antihypertensive drugs who came to the outpatient clinic between the month of November 1998 were stratified according to their sexes, ages and stage of hypertension before randomly assigned to the study or control groups. The pharmacist counseled the patients in the studied group for 5 time (6 months totally). There were 30 patients in each group, 20 females and 10 males. Meanages in the study and control groups were 63.6 and 62.3 years old, respectively. At the end of the studied period, 60% of the patients in the studied group had decreasing mean arterial pressure (10 mmHg or more) while only 40% of the control group exhibited this. Out of 60% of the patients in the studied group whose blood pressure changed, only 20% had the treatment regimen changed. The mean score of the knowledge about hypertension and treatment of the studied group before and after counseling were 36.5 and 43.8 respectively, which showed significantly increased (P<0.05). Noncompliance rate also declined from 43.3% to 10.0% after pharmacist counseling. Six adverse drug reaction probjems were found. The pharmacist solved 1 problem and prevented the patients to stop taking their medicine for 4 problems. Fifty seven potential drug interactions which required closely monitoring were identified. In average, the pharmacist spent 4.6 minutes on each counseling which equivalent to the cost of 10.1 baths. The studyshowed that pharmacist counseling on ambulatory hypertensive patients was very useful. It helps decreasing the patients' blood pressure, increasing the patients' knowledges about hypertension and treatment, decreasing noncompliances and most important it helps the patients to have a better control of their blood pressure. It should therefore be implemented in every hospital for the benefit of the patients.