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To study the influence of seasons on blood pressure in ordinary circumstances.We examined seasonal variations of home and 24 h ambulatory and office blood pressures in outpatients with essential hypertension. Office, home and ambulatory blood pressures of 50 outpatients with essential hypertension were recorded in 1993. The subjects were 26 women and 24 men, aged 59.3 ± 1.1 years (mean ± SEM). Office blood pressure was measured monthly by physicians. Home blood pressure was measured every day by the patients in the morning and evening. Ambulatory blood pressure was recorded every 30min in summer and in winter. The order of ambulatory blood pressure monitoring was randomized. The daytime and night-time blood pressures were calculated according to the true waking and sleeping times of the individual patients.Both office and home blood pressures showed significant seasonal variations. The winter-summer differences in office and home blood pressures were 4.7 ± 1.3/3.3 + 0.9 and 5.9 ± 1.1/2.7 + 0.6 mmHg, respectively. They were not influenced by the presence of antihypertensive agents. The winter-summer difference was also significant for daytime ambulatory blood pressure (3.5 ± 1.4/2.5 + 0.8 mmHg), but not for night-time ambulatory blood pressure (-2.9 + 1.7/-1.2 ± 1.0 mmHg) or average 24 h blood pressure (1.5 + 1.3/1.2 + 0.7 mmHg). There were no significant differences in the waking and sleeping times between the two seasons.Office, home and daytime ambulatory blood pressure levels were higher in winter than they were in summer in patients with essential hypertension. However, the seasonal variations in average 24 h blood pressure may be small because of the lack of changes in night-time blood pressure.