Two types of passive samplers, SPMDs and POCIS, were deployed in March 2006 at seven sties in Lake Mead National Recreational Area (Nevada and Arizona) to help assess sources of endocrine disrupting compounds (EDCs). Extracts were analyzed by GC/MS using two detection methods, electron capture, and negative capture ion. A total of 47 compounds were detected, 19 of which are known or suspected EDCs. The major source of EDCs was Las Vegas Wash, which consists of tertiary treated effluent, surface runoff from the Las Vegas Valley, and groundwater discharge. A commonly used polycyclic musk fragrance, tonalide, was found at considerably higher levels in both SPMDs (260 µg) and POCIS (9.8 µg) deployed in Las Vegas Wash compared to other EDCs, followed by another musk fragrance, gaxolide. Both of these compounds were also detected downstream from Las Vegas Wash in Lake Mead and tonalide was detected further out in Boulder Basin but not below Hoover Dam or at the two major inflows to Lake Mead, Overton Arm and Gregg Basin. Polycyclic musks have been shown to be only weakly estrogenic, however, they are present in Las Vegas Wash and Bay at orders of magnitude higher than levels of other EDCs, including many legacy organochlorine compounds. Another estrogenic EDC commonly present in treated wastewater as a surfactant, octylphenol, was detected in Las Vegas Wash and Bay samples. The presence of these estrogenic EDCs is supported by evidence of endocrine disruption in male fish collected from the Las Vegas Wash and Bay, and below Hoover Dam. Effects include altered sex steroid hormones, reduced gonadal development, and lower sperm quality suggesting exposure to estrogenic EDCs.