On November 25, 2008, the U.S. Department of Health and Human Services posted a final notice in the Federal Register authorizing the use of liquid chromatography-tandem mass spectrometry (LC-MS-MS) and other technologies in federally regulated workplace drug testing (WPDT) programs. To support this change, it is essential to explicitly demonstrate that LC-MS-MS, as a technology, can produce results at least as valid as gas chromatography (GC)-MS, the long-accepted standard in confirmatory analytical technologies for drugs of abuse. A series of manufactured control urine samples (n = 10 for each analyte) containing amphetamine, methamphetamine, (±)-3,4-methylenedioxyamphetamine, (±)-3,4-methylenedioxymethamphetamine, (±)-3,4-methylenedioxyethylamphetamine, phencyclidine, and (±)-11-nor-9-carboxy-Δ⁹-tetrahydrocannabinol at concentrations ranging from 10% to 2000% of federal cutoffs were analyzed with replication by five federally regulated laboratories using GC-MS and at RTI International using LC-MS-MS. Interference samples as described in the National Laboratory Certification Program 2009 Manual were analyzed by GC-MS and LC-MS-MS as well as previously confirmed urine specimens of WPDT origin. Matrix effects were assessed for LC-MS-MS. Results indicated that LC-MS-MS analysis produced results at least as precise, accurate, and specific as GC-MS for the analytes investigated in this study. Matrix effects, while evident, could be controlled by the use of matrix-matched controls and calibrators with deuterated internal standards.