Fentanyl and its analogues are commonly named synthetic opiates. Some compounds from fentanyl group (e.g. fentanyl, alfentanil, sufentanil, remifentanil) are used as anaesthetic and analgesic of high potency and short duration of action. Effects of acting fentanyls are indistinguishable from those produced by nasal inhalation of street heroin. In view of this, fentanyls are very "attractive" for the narcotic market. In least years an increase in number of reports about illegal production and non-medical use of fentanyl and its analogues have appeared. Numerous fentanyl analogues are sold under one name synthetic heroine or China white. Fentanyl and its analogues also can be used as gas warfare agents. One of fentanyl-related compound was used during action of easing hostages from Dubrovka theatre. Because of respiratory depression, which was caused by used compound, died over 100 hostages. Because of high potency fentanyl-related compounds are used in very low doses. So fentanyls appear in biological material in very low concentrations, not higher than several nanograms per millilitre or per gram. The drugs in these concentrations cannot be detected by means of routinely used screening procedures. Therefore direct methods for determination of this group of compounds are needed. Screening method for determination of fentanyl and its analogues was elaborated and validated. Liquid chromatography-mass spectrometry (LC/MS) technique was applied. The method is characterised by the limit of quantification (LOQ) and the limit of detection (LOD) ranged from 0.6 to 2 ng/ml and from 0.2 to 0.6 ng/ml, respectively for four above-mentioned compounds. The method was used for determination of fentanyl in three forensic cases. The blood, bile and blood from lung samples were taken during autopsy of persons who died shortly after surgical procedure in which fentanyl was used as an adjunct to general anaesthesia.