To determine the factors that affect seasonal variation in blood pressure (BP) in a fairly large number of patients with essential hypertension who stayed almost entirely indoors in a stable environmental temperature and who took a calcium channel blocker during the study. This prospective study of hypertensive patients was conducted during the summer and winter. BP was measured by ambulatory blood pressure monitoring; the indoor temperature was measured at the time of the BP measurement using an electrothermometer with the subject awake and indoors. Subjects comprised 38 men and 57 women. The subjects spent virtually the entire day indoors during both the summer (men, 22.1±1.6h; women, 23.0±0.9h) and winter (men, 23.0±0.9h; women, 22.9±0.9h). During the waking hours, the systolic/diastolic BPs were significantly higher during the winter than the summer. The differences were 8±9/4±5mmHg in men and 5±11/2±6mmHg in women; these values were not significantly different between men and women. No significant seasonal differences in BP during the sleeping hours were noted. There was a significant difference of approximately 6°C in the environmental temperature during waking hours, but there was no significant difference in urinary excretion of sodium or in exercise activity between the seasons. Only body height was significantly correlated with the winter increase in waking BP in both men and women, even after adjustment for potential confounding variables. Body height was a determinant of the increase in waking BP during the winter in hypertensive patients who lived almost entirely indoors. (Hypertens Res 2000; 23: 587-592)