To investigate whether the 24-hour blood pressure (BP) profile of primary aldosteronism differs from that of primary hypertension, ambulatory BP monitoring was performed in 11 patients with primary aldosteronism (9 with an adrenal adenoma and 2 with idiopathic hyperaldosteronism) and in 11 primary hypertensives, matched for sex (5M,6F), age (mean: 52 vs 49 yrs) and casual BP. We found no difference in 24-hour BP, nocturnal BP fall, BP variability (standard deviation and peaks of pressure) response to postural changes (lying-standing BP) between the two groups (all p values n.s.). Within the patients with primary aldosteronism no correlation was observed between BP, plasma renin activity, blood and urine aldosterone levels, blood and urine K+, and size of the tumour. Thus, at variance with previous reports, these results show that diurnal rhythm of BP and BP variability are similar in primary aldosteronism and primary hypertensives with similar demographic features and causal BP levels. They also show that an orthostatic fall of BP is not a common feature in this disease.