We propose a new confirmatory method for testosterone doping in sport. The present method in use, based on measuring the testosterone/epitestosterone (T/E) ratio in urine, may miss suspicious cases, or lead to reporting cases in which the high ratio is natural. Synthetic testosterone has a 13C abundance different from that of endogenous human testosterone. The connection of a gas chromatograph to an isotope-ratio mass spectrometer via a combustion interface allows the measurement of the corresponding characteristic value (delta /1000) for testosterone, its precursors, and its metabolites. To detect exogenous administration of testosterone, 30-40 mL of urine is sufficient.