We have previously reported that the urine of patients with Cushing's syndrome, including pituitary adenoma cases and adrenal adenoma cases, consistently show a conspicuous peak in the chromatographical analysis of 17-ketosteroid fraction but not in the urine of control subjects. The substance emerges just before 11beta-hydroxy-androsterone (11beta-OH-A) in capillary gas chromatography. In the present study, we have identified an "unknown peak substance" observed in the urine of Cushing's syndrome patients using gas chromatography-mass spectrometry (GC/MS). Trimethylsilylether (TMS)-derivative of the substance was found to have a molecular weight (MW) of 448, which is similar to that of 11-OH-A (MW: 450). From these findings, we hypothesized that the substance had the structure of a C-19 steroid with two hydroxyl groups at positions C-3 and C-11, one keto-group at C-17 and a double bond between C-4 and C-5 of the A ring. We hypothesized that the unknown peak substance was 3alpha,11beta-dihydroxy-4-androsten-17-one (3alpha,11beta-DH-A). To confirm this speculation we synthesized 3alpha,11beta-DH-A and compared the elution pattern of it with that of the "unknown peak substance" using GC and GC/MS. We found that both substances were indistinguishable by GC and GC/MS analysis. These results suggest that the unknown substance observed in the urine of patients with Cushing's syndrome is 3alpha,11beta-DH-A.