The characteristics and determining factors of seasonal variations of the blood pressure (BP) were studied in 20 normal subjects and 219 chronic stable chronically all patients, most of whom were ambulatory. The BP was measured repetitively over twelve months and measurements in the lying position repeated after one minute of orthostatism were performed in Winter and in Summer. In normal subjects, the BP decreased from June to reach its lowest value in August to return to the Winter values from October. The mean BP of the three Winter months was 130/79 mmHg and the seasonal lowering averages 5 +/- 5/5 +/- 6 mmHg (m +/- SD) (p < 0.01), with marked individual differences. The Summer decrease in BP was observed both lying down (3/4 mmHg) (p = 0.01) and standing (5/6) (p = 0.0001). In the patient group, the Summer decrease in BP was 4/3 mmHg. During the orthostatic measurements, it was 4/4 mmHg lying down and 6/5 mmHg when upright. Symptoms of orthostatic hypotension were reported spontaneously 10 times during the Winter and 21 times during the Summer months (p = 0.04) and occurred in the upright position in 12 patients (6%) in Winter and 25 patients (12%) in Summer (p = 0.025). The Summer decrease was greater in women than in men. Blood pressure lowering drugs increased this effect and the association of several drugs had an additive effect. It increased with age but disappeared after 70-80 years of age. The very elderly patients on antihypertensive therapy showed a marked decrease in BP during the Summer, especially in the upright position. The Summer decrease in BP is important for the management of elderly patients with hypertension or cardiac failure. It may favorize symptoms of orthostatic hypotension and increase the risk of malaise.