Both F(2)-isoprostanes (8-iso-PGF(2alpha)), a well-known marker of oxidative stress, and thromboxanes A(2) (TXA(2)) are involved in atherosclerosis through LDL oxidation and platelet activation. Different aspects of the pathology can be described by 8-iso-PGF(2alpha) and TXA(2) so it is important to determine both their concentrations to monitor the disease progression and/or therapy effects. We developed a simple and sensitive method based on liquid chromatography-tandem mass spectrometry, using electrospray ionization in negative-ion mode, for the simultaneous measurement of the concentration of 8-iso-PGF(2alpha) and 11-dehydro thromboxane B(2) (11-DH-TXB(2)), a TXA(2) metabolite. This method was applied to analyze urine samples collected overnight from 15 atherosclerotic patients, with documented carotid artery sclerosis (CAS), and from 20 controls. The detection limit was 0.097pg/microL for 8-iso-PGF(2alpha) and 0.375pg/microL for 11-DH-TXB(2), with a linear range of 0.78-25pg/microL; the inter- and intraday imprecision was <5% for both metabolites. These analytes were higher in CAS (P<0.005 vs controls) and were positively correlated in patients but not in controls, even after adjustment for age and gender (r=0.60; P=0.032). This highly sensitive, precise, and rapid method allows for the simultaneous determination of 8-iso-PGF(2alpha) and 11-DH-TXB(2) in human urine samples in order to evaluate oxidative stress and platelet aggregation.