Perfluorochemicals (PFCs) are an emerging class of organic contaminants of global concern. Perflorooctanoic acid (PFOA, also abbreviated C8) and perfluorooctane sulfonate (PFOS) are two primary PFCs. PFOA and PFOS are reportedly persistent in the environment and bioaccumulate with controversial adverse health effects to wildlife and people. According to USEPA's report, PFOA is a "likely" human carcinogen and can remain in human body for up to four years. Toxicity tests have raised concerns about potential developmental, reproductive, and systematic effects of PFOS. However, the harmful exposure levels are still in debate and further risk assessment is required to understand the toxicity of PFOA and PFOS. Measuring PFOA and PFOS concentrations in drinking waters requires a highly sensitive analytical method. One of major problems associated with trace level analysis of PFOA and PFOS is background contamination in the analytical blanks. The background may result from the Teflon parts that are often used in many analytical instruments, solvent generation and purification systems, and sample collection and preparation devices. This work presents a highly sensitive liquid chromatography/electrospray ionization/tandem mass spectrometry (LC/ESI/MS/MS) method, coupled with an effective solid-phase extraction (SPE) sample preparation technique, to meet the need for the measurement of trace level PFOA and PFOS in drinking waters. In this research, drinking water samples from different geographic regions throughout the United States were analyzed to provide contamination levels of PFOA and PFOS. The internal standard (IS) was 2,3,5,6-tetrafluoro-4- (pentafluorophenyl)phenol. The surrogate standard (SS) was 4'-trifluoromethyl)-2- biphenylcarboxylic acid. Sample preparation and analysis, accuracy, precision, and method detection limits are all discussed. Includes 8 references, tables.