GH secretagogues (GHS) have been used for the differential diagnosis of ACTH-dependent Cushing's syndrome (CS) since 1997 due to their ability to increase ACTH and cortisol levels in Cushing's disease. The aim of this study was to correlate ACTH response to GH-releasing peptide-6 (GHRP-6) in vivo with GH secretagogue receptor type 1a (GHSR-1a) mRNA expression in a patient with lung carcinoid tumor. The patient was a 26-yr-old male with diagnosis of ACTH-dependent CS. He presented negative responses to human CRH and desmopressin tests; yet, a significant increase in ACTH after the GHRP-6 test was observed. Sellar magnetic resonance imaging (MRI) showed slight posterior hypointensity, but bilateral petrosal sinus sampling did not show central gradient. Computed tomography (CT) and MRI of thorax/abdomen/cervical were negative and 111In-pentetreotide scintigraphy depicted abnormal uptake on the right lung. The patient was submitted to right thoracotomy for exeresis of lung nodule and hilar lymph node which were characterized as atypical lung carcinoid tumor and he presented clinical and laboratorial remission after surgery. GHSR-1a mRNA expression was studied with real-time quantitative PCR and tumor data were compared with fragments of normal lung and pituitary. There was a higher GHSR-1a expression in the lung carcinoid tumor as compared with normal tissues. The ACTH response to GHRP-6 in a patient with ectopic ACTH production by a lung carcinoid tumor was associated with GHSR-1a expression in the tumor tissue, suggesting an association between GHSR-1a mRNA overexpression and the in vivo response to GHS.