There is concern that wastewater treatment system workers are at risk for cancers and diseases affecting the neurological and digestive systems. However, these diseases have also been linked to early exposures. A proportional mortality study was conducted on a large cohort of wastewater treatment system workers who were divided into two groups, migrants and nonmigrants, by place of birth as reported on their death certificates. The migrant worker group was significantly higher than the US white male population for cancer of the stomach, leukemia, and all lymphopoietic cancers. Migrant workers also had an elevated ratio for all diseases of the nervous system and sense organs. No cases of amyotrophic lateral sclerosis were found. The American-born workers had an elevated rate of death for arteriosclerotic heart disease compared with the US white male population. We suggest that place of birth may present a confounding factor when evaluating exposures in employee groups.