The purpose of this study was to evaluate the circadian blood pressure rhythmicity in patients with primary adrenal insufficiency. Ten newly diagnosed and untreated patients with Addison's disease, 17 addisonian patients (including the previous 10) following an adequate regimen of corticosteroid replacement therapy, and 15 healthy subjects as a control group were studied. Twenty-four-hour ambulatory blood pressure and heart rate were measured automatically every 30 min using a Takeda TM-2420 recorder. The runs test, used for detecting blood pressure diurnal rhythm, was compatible with a rhythm in 2 of 10 patients with untreated Addison's disease, in 13 of 17 patients with treated Addison's disease, and in 13 of 15 normals. Six of eight addisonian patients showing no evidence of blood pressure rhythm in the untreated state acquired circadian periodicity during therapy. An analysis of blood pressure readings by Fourier series with four harmonics showed that blood pressure mesor was lower in untreated than in treated addisonian and normal subjects (P < .05). The nocturnal fall was smaller for systolic and diastolic blood pressure in untreated than in treated addisonian and normal subjects (P < .05). Adrenocortical insufficiency is often characterized by loss of circadian blood pressure rhythm, and normal rhythm can be reestablished by replacement therapy. Lack of cortisol rhythm or persistent activation of the renin-angiotensin system and sympathetic tone may play a role in this phenomenon.