BACKGROUND: Seasonal variation of blood pressure (BP) is well documented. It means that season should be considered in diagnosis and treatment of hypertension. This study was done to evaluate seasonal influence on blood pressure of hypertensive patients who had been taking the same antihypertensive medicine for a long time. METHODS: In medical records, we reviewed the blood pressure recordings of 328 hypertensive patients who had been taking the same medicines through the consecutive summer (Jun., Jul., and Sep.) and winter(Dec., Jan., and Feb.) period. We evaluated the blood pressure difference between summer and winter, and also evalauated influence of age, sex, BMI, smoking, alcohol drinking and exercise on seasonal difference of blood pressure. RESULTS: An average of four blood pressure readings per patient, two for summer and two for winter were recorded. Ambient temperature was over 22degrees C in summer and below 4degrees C in winter. The systolic blood pressure in summer and winter were 135.5mmHg and 140.5mmHg, respectively, and the diastolic blood pressure in summer and winter were 85.0mmHg and 87.4mmHg, respectively. Both systolic and diastolic blood pressure increased significantly in winter. Both in systolic and diastolic blood pressure, sex, smoking, alcohol drinking, exercise did not have an influence on seasonal difference. Age and BMI did not have an influence on seasonal difference of systolic blood pressure. But in diastolic blood pressure, age and BMI had an influence on seasonal difference and elderly patients had a greater seasonal difference in diastolic blood pressure than younger patients. CONCLUSIONS: In hypertensive patients who had taken the same antihypertensive medicines for a long time, both systolic and diastolic blood pressure was significantly creased during the winter. Therefore season should be considered in the treatment of hypertension even though patients are stable with the same medicines.