Two distinct subtypes of patients with primary aldosteronism due to aldosterone-producing adenomas (APA), based on different aldosterone responses to angiotensin, have been identified. We evaluated the relationship between adrenal zona fasciculata-like histotype and response of plasma aldosterone to upright posture in a series of patients with APA. Twenty-five patients were retrospectively divided in two groups according to aldosterone response to posture, i.e., a first group without postural change of aldosterone (n = 19) and a second group with at least 30% aldosterone increase after standing (n = 6). The percentage of zona fasciculata-like cells was calculated at histology in all adenoma tissues removed at adrenalectomy. The two groups of patients were similar in sex, age, systolic/diastolic blood pressure, supine/upright plasma renin activity, supine/upright aldosterone, tumor size. No differences between the two groups were observed as to zona fasciculata-like (84 +/- 3% vs 71 +/- 9%, P NS) and non-zona fasciculata-like cells percentage in adenoma tissues. No inverse correlation was found in either group between the percentage change from supine to upright aldosterone and the percentage of zona fasciculata-like cells. Aldosterone and cortisol responses to ACTH testing were similar in the two groups. Our results indicate that the two subtypes of primary aldosteronism based on different postural responses of aldosterone are not due to a different prevalence of zona fasciculata-like histotype in APA.