To assess the influence of environmental temperature on blood pressure, we analyzed 42,813 readings of systolic and diastolic blood pressures in recumbent and upright positions in 2,000 hypertensive patients in our clinic in correlation to noon temperature and mean daily temperature in Montréal. The sample comprised 860 men and 1140 women with an average age of 55 years. Analysis of variance showed highly statistically significant negative correlations for all blood pressure values between different noon temperature, as well as mean daily temperature, groups. The greatest difference for systolic blood pressure was 7 mm Hg, and for diastolic blood pressure it was 3 mm Hg, within the -24 degrees C to 27 degrees C temperature range. The same relationship between blood pressure and different environmental temperatures was also seen when the blood pressure recordings were divided between temperature below 0 degree C and above 0 degree C, ie, within the winter or summer months. This environmental effect may have a clinically significant impact because thermosensitivity appears to be related to hypertension.