The hexachlorocyclopentadiene (77474) (HCCPD) contamination of a waste water treatment facility and its effects on workers were described. Although the workers wore protective clothing, they reported irritation of the eyes and skin, sore throat, cough, headache, chest discomfort, nausea and fatigue. Workers were given questionnaires, physical examination, blood and urine tests. Some employees reported symptoms for 6 weeks after the contamination incident. Laboratory tests showed no significant abnormalities in renal function, complete blood counts, or urinalyses. Some abnormalities were found in liver function. The airborne concentrations of HCCPD and related chlorinated hydrocarbons could not be determined but results from this exposure provide evidence of the potential toxicity of HCCPD. The authors suggest that municipal sewage systems install a monitoring system to detect chemical contamination before employees are exposed.