Buprenorphine is a strong narcotic analgesic. It is also used in the substitution therapy for opium alkaloid addicts. The aim of this paper was to develop and validate a highly sensitive high-performance liquid chromatography-electrospray ionization-mass spectrometry method for simultaneous determination of buprenorphine and norbuprenorphine in human serum. The developed methodology was then applied to real clinical cases in a clinical toxicology setting. Extraction of analytes has been done using solid-phase extraction. Chromatographic separation was achieved on a LiChroCART column with a Purospher RP-18e cartridge, and for detection an LCQ mass spectrometer with an ion trap analyzer was used. Quantitation of buprenorphine and norbuprenorphine was performed in a single ion monitoring mode (m/z 468 buprenorphine, m/z 414 for norbuprenorphine) in order to increase the sensitivity of the method. The standard curves for both compounds were linear over the range of 0.2-10 ng/mL (r2 > 0.995). The quantitation limit was 0.2 ng/mL for both analytes. The method was used for quantitation of both buprenorphine and norbuprenorphine in the serum of 15 patients undergoing the buprenorphine substitution therapy. Serum concentrations ranged between 0.36 and 4.60 ng/mL for buprenorphine and 0.21 and 2.50 ng/mL for norbuprenorphine, with buprenorphine single dosages from 0.8 to 6.0 mg.