Preoperative localization of ACTH-secreting microadenomas has been performed in 9 patients with Cushing's disease by using bilateral and simultaneous venous sampling of the inferior petrosal sinuses. In addition to ACTH and PRL we determined GH levels after oCRH stimulation in order to confirm the possible occurrence of unilateral GH increases, as recently observed by us in one patient. A central-to-peripheral and an intersinus gradient of ACTH concentration was observed in all patients examined before and/or after oCRH stimulation. In 7 patients central-to-peripheral and side-to-side PRL gradients were recorded in basal conditions; in 5 of these patients a unilateral oCRH-induced PRL increase was observed. Six out of the 7 patients with unilateral PRL increases also showed an intersinus GH gradient in basal conditions (ratio ≥ 1.5); in 5 of them a clearcut oCRH-induced GH increase was observed. A peripheral oCRH-induced GH and PRL increase was not observed in any of the patients. The observation of a paradoxical oCRH-induced GH increase in the inferior petrosal sinus with the higher ACTH concentration is of speculative and clinical interest; whether it reflects co-secretion of hormones by the tumour or hormone release by non-tumourous cells via paracrine mechanism is still to be clarified. The unilateral GH increase could represent an additional signal of the presence and localization of an ACTH-secreting tumour.