INTRODUCTION: Diagnosis and differential diagnosis of Cushing's syndrome (CS) remain considerable challenge in endocrinology. For more than 20 years, CRH has been widely used as differential diagnostic test. Following the CRH administration, the majority of patients with ACTH secreting pituitary adenoma show a significant rise of plasma cortisol and ACTH, whereas those with ectopic ACTH secretion characteristically do not. OBJECTIVE: The aim of our study was to assess the value of CRF test for differential diagnosis of CS using the ROC (receiver operating characteristic) curve method. METHOD: A total of 30 patients with CS verified by pathological examination and postoperative testing were evaluated. CRH test was performed within diagnostic procedures. ACTH secreting pituitary adenoma was found in 18, ectopic ACTH secretion in 3 and cortisol secreting adrenal adenoma in 9 of all patients with CS. Cortisol and ACTH were determined -15.0, 15, 30, 45, 60, 90 and 120 min. after i.v. administration of 100 microg of ovine CRH. Cortisol and ACTH were determined by commercial RIA. Statistical data processing was done by ROC curve analysis. Due to small number, the patients with ectopic ACTH secretion were excluded from test evaluation by ROC curve method. RESULTS: In evaluated subgroups, basal cortisol was (1147.3 +/- 464.3 vs. 1589.8 +/- 296.3 vs. 839.2 +/- 405.6 nmol/L); maximal stimulated cortisol (1680.3 +/- 735.5 vs. 1749.0 +/- 386.6 vs. 906.1 +/- 335.0 nmol/L); and maximal increase as a percent of basal cortisol (49.1 +/- 36.9 vs. 9.0 +/- 7.6 vs. 16.7 +/- 37.3%). Consequently, basal ACTH was (100.9 +/- 85.0 vs. 138.0 +/- 123.7 vs. 4.8 +/- 4.3 pg/mL) and maximal stimulated ACTH (203.8 +/- 160.1 vs. 288.0 +/- 189.5 vs. 7.4 +/- 9.2 pg/mL). For cortisol, determination area under ROC curve was 0.815 +/- 0.083 (CI 95% 0.652-0.978). For cortisol increase cut-off level of 20%, test sensitivity was 83%, with specificity of 78%. For ACTH, determination area under ROC curve was 0.637 +/- 0.142 (CI 95% 0.359-0.916). For ACTH increase cut-off level of 30%, test sensitivity was 70%, with specificity of 57%. CONCLUSION: Determination of cortisol and ACTH levels in CRH test remains reliable tool in differential diagnosis of Cushing's syndrome.