An incident involving unauthorized discharge of hexachlorocyclopentadiene (77474) (HCCPD) into municipal sewage was investigated. On March 26, 1977, large amounts of HCCPD entered a sewage treatment facility in Kentucky causing immediate tracheobronchial irritation among workers and requiring them to seek medical attention. Water analysis showed additional contamination with octachlorocyclopentene (706785) and traces of other chlorinated cyclohydrocarbons. Air samples obtained 4 days after worker exposure showed HCCPD concentrations between 270 and 970 parts per billion (ppb); the 8 hour time weighted average limit recommended by the American Conference of Governmental Industrial Hygienists was 10ppb. A group of 193 employees who had worked at the sewage facility for 2 or more days between March 14 to 29 were sent a questionnaire seeking information on demographic background, work history, personal habits, symptoms, and occupational sources of chemical exposure. Workers who had complaints of skin or mucous membrane irritation were examined and biological samples were taken for screening. The most frequent symptoms included eye irritation (59 percent), headaches (45 percent), and throat irritation (27 percent). Skin irritation, cough, nausea, and abdominal cramps were also reported. Odor detection preceded the onset of symptoms in 94 percent of workers. As long as 6 weeks following exposure, workers still reported fatigue, respiratory tract irritation, headaches, and eye irritation. Elevated lactate-dehydrogenase levels were noted in 11 of the 41 workers who were tested and proteinuria was detected in six. The authors conclude that increased exposure of sewage workers to hazardous chemicals will continue to expand as larger and larger volumes of industrial effluents are channeled through municipal wastewater treatment facilities.