The antiparkinsonian drug trihexyphenidyl (THP) is currently manufactured and administered as a racemate. However, stereochemistry can play significant role in the drug's pharmacokinetics, biotransformation, metabolism, interaction with cellular and tissue components and overall effect on human body. It is necessary to consider such a drug as a mixture of two compounds (drug enantiomers), with their own effect on the human body. The present paper describes a simple and sensitive LC-MS method for the stereoselective determination of THP in human serum. In this study, the sample was prepared by a solid-phase extraction (SPE) procedure. The enantiomer separation was done using native beta-cyclodextrin stationary phase LC column. The combination of ESI-MS detection and SPE showed excellent sensitivity and selectivity of the method. The limits of detection of <0.1 ng/ml can be easily achieved, which is 7,000 times lower than the detection limits achievable by a UV detection method. The method has at least a 3-order of magnitude linear dynamic range for both enantiomers (concentrations up to 1,323 ng/ml were tested). This is 24 times wider than the therapeutic range of THP (peak THP plasma concentration of 55 ng/ml was previously reported). The recoveries of THP enantiomers from the human serum were > 95%.