This case study involves the structure elucidation of the designer steroid tetrahydrogestrinone or THG by the UCLA Olympic Analytical Laboratory. This steroid was allegedly synthesized by BALCO organic chemist Patrick Arnold and was allegedly used by the famous baseball player, Barry Bonds. Students will learn the logic of structure elucidation and the techniques used by chemists to determine molecular structure. Students are asked to make predictions with respect to the changes in the proton NMR upon conversion of gestrinone to tetrahydrogestrinone. The case was originally developed for use in an organic chemistry survey course for non-chemistry majors, but could also be used in a general/organic/biochemistry survey course or modified for use in an organic chemistry sequence. Two sets of questions are included; the first is appropriate for students who are able to interpret skeletal structures and have studied addition reactions to alkenes, including hydrogenations, while the second is more advanced. Students will need access to a 2004 paper by D.H. Catlin et al. published in Rapid Communications in Mass Spectrometry (not included in this case).