A sensitive method is described to detect isolysergic acid diethylamide (iso-LSD) in urine. The compound was extracted from urine and converted to a C-8 carbanion by sodium ethoxide in ethanol. Protonation of the carbanion by water selectively produced LSD. The conversion of iso-LSD to LSD was almost quantitative (98%). The product was purified by solid-phase fractionation and acid-base separation techniques. The trimethylsilyl derivative of LSD was detected by a gas chromatography-mass spectrometry method. The overall recovery of the procedure was approximately 69%. Quantification of iso-LSD was linear over the concentration range 50-2000 ng/L. In specimen analysis, iso-LSD was detected when the LSD concentration was below the limit of the detection (50 ng/L) of the procedure. Because iso-LSD is a byproduct of illicit preparation of LSD, presence of iso-LSD in urine is an indication of LSD use.