The gold standard for the diagnosis of acute poisoning is toxicological analysis. Because information on the incorporated toxic substance provided by the patient or his relatives is known from experience to be unreliable in about 40% of all intoxications, a screening procedure that covers most relevant drugs and toxicants is required rather than an analytical procedure optimized for the identification of a single class of substances. The special task for a general unknown screening procedure is to identify a toxic substance among endogenous or food-derived substances as well as environmental toxicants in a biological matrix on an emergency basis. Because the unknown toxic substance may vary considerably in its physicochemical properties and its concentration range, a universally applicable screening procedure is required. Although gas chromatography-mass spectrometry has been used for three decades, it still offers many unique advantages in terms of sensitivity, specificity, reliability, and coverage of a large number of toxic substances. Because the procedure has to be kept as simple and as short as possible, compromises have to be made with respect to extraction, derivatization, and mass-spectral techniques. The specimen of choice for a general unknown screening is-if available-urine. The standard mode of ionization is electron impact. The identification of unknown substances is highly challenging because, in our experience, previously unknown metabolites may be detected rather frequently in acute poisoning. Although an automated mass spectra library search considerably facilitates the identification process, expert knowledge on the identification of substances not included in the library as well as knowledge in clinical toxicology and metabolism is indispensable.