We investigated the seasonal changes in blood pressure (BP) and in short-term BP variability determined using ambulatory blood pressure monitoring (ABPM). 1000 white subjects, who took part in the multicenter HARVEST study, underwent ABPM with the A&D TM-2420 or the Spacelabs 90207. Standard deviation of the mean daytime and nighttime BP was taken as an index of short-term BP variability (v). Maximal outdoor temperature (Tmax) during each ABPM was obtained from local Meteorological Centers. Subjects were divided according to season and to quartiles of Tmax. A subgroup of 46 persons who repeated ABPM in Winter and Summer was also studied. We observed evident seasonal differences in office and ambulatory systolic BP (SBP) with a peak during Winter. Diastolic BP (DBP) and heart rate did not vary throughout the four seasons. Office SBP (p < 0.01), 24-hour (p < 0.002), daytime SBP (p < 0.0001), both daytime SBPv (p < 0.0001), DBPv (p < 0.02), and nighttime SBPv (p < 0.05), DBPv (p < 0.02) as well as norepinephrine (p < 0.005) were significantly higher during Winter than Summer. Similar differences were observed in subjects grouped in quartiles of Tmax. In the subgroup daytime but not nighttime SBP was higher in the cold season. Average 24-hour SBP (p < 0.05), daytime SBP (p < 0.02), daytime SBPv (p < 0.001) and DBPv (p < 0.05) and norepinephrine (p < 0.0001) were significantly negatively correlated with Tmax in the whole population. BP is higher and subjected to wider oscillations during the cold season in patients with mild hypertension probably due to sympathetic activation. The assessment of a hypertensive subject may give different results according to the season.