Stanozolol, a synthetic anabolic androgenic steroid, is often abused in sports to enhance performance. Consequently, the anti-doping laboratories daily screen for its metabolites (3'hydroxystanozolol and 4beta hydroxystanozolol) in all urines, mainly by GC-MS, after enzymatic hydrolysis and TMS derivatization. A sensitive and specific method by GC-MS3 has been developed for the identification in urine of 3'hydroxystanozolol at trace levels. Full mass spectra and diagnostic ions are presented and a case report is commented. In this case, it was possible to attest the presence of a low concentration of stanozolol metabolite in a sample obtained from a competition test. This would have not been possible with other analytical techniques used in the laboratory. Through this case report, it was also possible to demonstrate the importance of sampling and the difficulties that has to face the laboratory when the pre-analytical step is not correctly performed.