Nine people living in Rhode Island, New York, Pennsylvania, Florida, or California acquired autochthonous brugian filariasis. Each patient had an enlarged lymph node containing a single worm or, in one patient, a pair of worms. Most worms were in lymphatic vessels within the node, but two worms were in the substance of the node. Ten worms were studied, seven female and three male. Female worms contained paired uteri that occupied most of the body cavity of the worm, and male worms contained a single reproductive tract. No worms were gravid. The diameter of the worms was small, 30 µm to 75 µm. The usual diameter of female worms was 65 µm to 75 µm, and 45 µm to 50 µm for male worms. The morphologic features of these worms, their anatomical location, and their geographic distribution are all characteristic of infection with a North American Brugia species.