Fifty randomly selected workers in a sewage treatment plant using a heat treatment method for processing sewage sludge in Toronto, Canada were assessed. This investigation revealed that many workers reported "influenza-like" symptoms, cough, sputum production, wheezing, sore throat and skin complaints. The workers tended to have somewhat reduced lung function. Workers in the area of the plant where boiled sewage sludge was dried frequently reported an intermittent, acute illness characterized by cough, fever and sore throat. Workers in the area of the plant where the dried sludge was incinerated tended to have reduced lung function. The basis for the possible health effects demonstrated has not been established, and further investigation is suggested. The mean level of polychlorinated biphenyls (PCBs) detected in the workers' serum was 6 +/- 5 ppb. The PCB levels could not be related to symptoms or clinical findings in the workers studied.