A method for the simultaneous measurement of monomethylarsonic acid (MMA) and dimethylarsinic acid (DMA) in human urine using liquid chromatography-tandem mass spectrometry with electrospray ionization (LC-ES-MS-MS) was developed. The multiple reaction monitoring mode (MRM) was used for quantitation. The protonated molecule ions (m/z 141.0 for MMA and m/z 139.0 for DMA) were selected as precursor ions, and the same fragment ion AsO+ (m/z 91.1) was monitored as the product ion. A two-step liquid-liquid extraction of MMA and DMA from urine provided recoveries of 92-100%. The coefficients of variation were lower than 7% for the within-day precision and lower than 11% for the between-day precision. The limit of quantitation was 25 microg/L as As for the two analytes. The assay was linear over the range of 25-800 microg/L.