A sensitive and specific high-performance liquid chromatographic-electrospray ionization tandem mass spectrometric (HPLC-ESI-MS-MS) method for quantification of tamsulosin in human plasma, using propranolol as internal standard (IS), has been developed, validated successfully, then used in a clinical study. Plasma (0.5 mL) was mixed with 50 μL 1 m sodium carbonate solution. Tamsulosin and propranolol were isolated from the mixture by liquid-liquid extraction with 7:3 ( v/ v) hexane-ethyl acetate. Reversed-phase chromatography was performed on a C 8 column at 25°C with 70:30:0.1 ( v/ v) methanol-water-formic acid as mobile phase at a flow-rate of 1.0 mL min −1. Quantification was achieved in positive-ion mode by monitoring the product ions at m/ z 409.1 → 270.9, 228.0, and 200.0 (tamsulosin) and m/ z 260.1 → 183.0 (IS). The lowest limit of quantification was 0.25 ng mL −1, and the calibration range was 0.25–50 ng mL −1. Within and between batch precision (expressed as coefficient of variation, CV) did not exceed 10.8% and accuracy was within 5.0% deviation of the nominal concentration. Recovery of tamsulosin from plasma was >83.0%. The validated method was used for clinical study of tamsulosin in human volunteers.