A high performance liquid chromatography (HPLC) method was developed and validated for determination of flunitrazepam in human plasma. After a simple liquid-liquid extraction, the analyses were carried out on a ODS column with diode array detection at 330nm. The mobile phase consisted in a mixture of potassium dihydrogene phosphate/acetonitrile (40/60, v/v). The method showed good linearity, accuracy and precision. Advantages of this validated assay include a simple plasma extraction method, short analysis time and good sensitivity (LLOQ = 5ng/mL). The stability data indicated a potential instability of flunitrazepam in plasma at room temperature.