Blood pressure (BP) was measured once every month during one year in 80 hypertensive outpatients. An orthostatic test was performed in winter and another during summer. The collective was aged 65 +/- 13 years (m +/- SD) and presented an elevated prevalence of diabetes mellitus, cardiac failure, and coronary, cerebral and arterial insufficiency. Diuretics, betablockers, converting enzyme inhibitors and calcium channel blockers were used by 31 patients as monotherapy and by 49 patients in association. During summer a significantly lower seated BP was found (144.1/79.4 mm Hg vs 150.8/82.9 mm Hg in winter, p < 0.001). One fourth of the patients did not show this diminution. On the basis of the WHO criteria of BP definition, 31% of the patients could be considered hypertensive in winter vs 16% in summer and 28% as normotensive in winter vs 43% in summer (p < 0.05). The summer reduction in BP depended on position. It was less marked in seated position (-5.3/-2.7 mm Hg) than in lying (-6/-5.1 mm Hg) or even in standing position (-10.8/-5.1 mm Hg). The orthostatic test induced a greater immediate fall in systolic BP in summer than in winter (-14.4 vs -9.6 mm Hg, p < 0.001), more orthostatic hypotensive episodes defined as a systolic BP fall of 20 mm Hg or more (34% of patients vs 20% in winter, p = 0.05) and more signs of reduced cerebral perfusion (14% vs 7.5% in winter, NS). Diabetic patients and patients treated by diuretic and vasodilator drugs are particularly exposed to orthostatic hypotension in summer. Betablockers can minimize this risk.(ABSTRACT TRUNCATED AT 250 WORDS)