Chlorine and potassium cresylate spilled from a train wreck forced evacuation of nearly 1,000 people in and near the town of Alberton, Montana, in 1996. Because respiratory and other symptoms persisted in this population, neurobehavioral and pulmonary functions were evaluated in a cohort of exposed vs. unexposed individuals. Ninety-seven subjects were tested 7 wk after exposure. Three years later, 36 of the original subjects were retested, along with 21 new patients exposed in the same incident. These 57 were compared with 22 unexposed individuals. Twenty-six neurobehavioral functions were tested, and spirometry was performed on each subject. At 7 wk postexposure, patients showed significant differences in 5 neurobehavioral functions (i.e., balance, simple reaction time, abnormal visual quadrants, vocabulary, and information), compared with the unexposed individuals recruited in 1999. Patients' Profile of Mood States scores and frequencies of 35 symptoms were also elevated, compared with the unexposed group. At 3 yr postexposure, patients exhibited differences in 7 additional neurobehavioral functions (i.e., choice reaction time, balance with eyes open, color errors, visual fields, Culture Fair, and verbal recall). Respiratory symptoms were increased, but pulmonary functions did not change. Exposure to chlorine and potassium cresylate produced neurobehavioral impairments that have been observed to increase across 3 yr. Spills in heavily populated areas could injure thousands, overwhelming medical facilities.