The most common lesion in Cushing’s disease is an anterior pituitary adenoma. However, normal or hyperplastic corticotropic pituitary tissue has also been found in some cases. In an attempt to distinguish the patterns of ACTH response to oCRH in different forms of anterior pituitary hypersecretion, 17 patients with pituitary adenoma and 17 without pathological evidence of adenoma were studied. These patients underwent transsphenoidal pituitary surgery by the same surgeon and were retrospectively evaluated. The diagnosis of pituitary lesions was confirmed by microscopical and immunohistochemical studies. Patients without pituitary adenoma showed a higher and more prolonged mean plasma ACTH response than that observed in patients with pituitary tumors. In patients with pituitary adenoma, the peak ACTH response was observed within 30 min after oCRH administration, and was followed by a gradual decrease to basal levels in the following 30 min. In those cases in whom no pituitary adenoma was found, oCRH injection produced a marked increase in plasma ACTH levels during the first 60 min with a slower decline at the subsequent time points. The mean response curves of the two groups, analyzed by Beherens-Fischer nonparametric ANOVA, showed significant differences, either when they were compared globally (p < 0.01), or at single time points. Differences in ACTH response to oCRH stimulation support the hypothesis of different pathogenetic mechanisms leading to ACTH hypersecretion in Cushing’s disease with and without pituitary adenoma.