A sensitive and selective liquid chromatography-tandem mass spectrometric (LC-MS/MS) method was developed and validated for the determination of adefovir (PMEA) in human serum and urine. The analyte was separated on a Diamonsil C(18) column (250 mm x 4.6 mm i.d., 5 microm particle size) by isocratic elution with methanol-water-formic acid (20:80:0.1, v/v/v) at a flow rate of 0.6 ml/min, and analyzed by mass spectrometry in multiple reaction-monitoring mode. The precursor-to-product ion transitions of m/z 274-->162 and m/z 226-->135 were used to measure and quantify the analyte and internal standard (I.S.), respectively. The weighted (1/x(2)) calibration curve was linear over serum concentration range 1.25-160.00 ng/ml and urine concentration range 0.05-8.00 microg/ml, with a correlation coefficient (r) of 0.9992 and 0.9978, respectively. The lower limit of quantification in human serum was 1.25 ng/ml. The inter- and intra-day precisions (R.S.D.%) in both serum and urine were lower than 8.64%, the mean method accuracies and recoveries from spiked serum samples at three concentrations ranged from 96.3 to 102.0% and 56.5 to 59.3%, respectively. The serum extract was stable when stored for 24h. The developed method was successfully applied to determine PMEA in human serum and urine, and proved suitable to clinical pharmacokinetic study.