AbstractThe administration of synthetic steroid copies is one of the most important issues facing sports. Doping control laboratories accredited by the World Anti‐Doping Agency (WADA) require methods of analysis that allow endogenous steroids to be distinguished from their synthetic analogs in urine. The ability to measure isotope distribution at natural abundance with high accuracy and precision has increased the application of Gas Chromatography–Combustion–Isotope Ratio Mass Spectrometry (GC–C–IRMS) to doping control in recent years. GC–C–IRMS is capable of measuring the carbon isotope ratio (δ13C) of urinary steroids and confirm their synthetic origin based on the abnormal 13C content. This tutorial describes some of the complexities encountered by obtaining valid δ13C measurements from GC–C–IRMS and the need for careful interpretation of all relevant information concerning an individual's metabolism in order to make an informed decision with respect to a doping violation. Copyright © 2008 John Wiley & Sons, Ltd.