The patient informed us that she had an identical twin. The identical twin was examined by the same group of 3 movement disorders specialists (J.H.F., M.E.T., and H.H.F.) who had seen the affected twin, about 9 months later, and the identical twin was healthy. Eighteen months after the initial contact, she reported over the telephone that she was still healthy. She was also genetically tested, and found monozygotic (MZ) with the same CAG repeat numbers in the HD gene. The asymptomatic twin had hypertension but none of the other disorders of her sister. She had stopped smoking cigarettes at the age of 35, whereas the twin with manifest HD had stopped at the age of 65, after her chronic obstructive pulmonary disease had developed. There were no differences in childhood illnesses, alcohol use, prescribed drug use as children, or caffeine intake. The birth home was across the street from a factory that produced large machines used to manufacture precision cutting tools from the late 1800s to 1987. “Numerous chemical spills and leaks occurred over the years.”1 The air was described as polluted, and in 1993, 2 years after the factory closed for the second time, the site was designated a federal toxic site requiring cleanup. It was considered primarily a “brownfield” site, containing mainly oil products, and the principal chemicals of concern identified were dichloroethene, trichloroethylene, and vinyl chloride. Both twins had been equally exposed to the factory’s toxins until the age of 23, when the asymptomatic twin moved away. The twin with manifest HD remained in her original house. The asymptomatic twin moved about 3.2 km away, within the same neighborhood, in New Bedford, Mass. Although the factory closed first in 1985, it was reopened from 1988 to 1991, and samples of air and soil are still analyzed by health department authorities, with reports provided to neighbors. The asymptomatic twin lives sufficiently far away that the air and soil in her neighborhood are not tested. A dietary/nutritional, lifestyle, and medication intake interview was performed, showing no significant differences.