The purpose of this study was to determine whether there was an association between occupational exposure to electromagnetic fields (EMFs) or polychlorinated biphenyls (PCBs) and mortality from prostate cancer among US electric utility workers. Data on participants, who were current and former employees of five large US electric utility companies, had been collected during 1987-1994, and the mortality of the cohort was followed through 1988. This nested case-control study contained 387 cases, men whose underlying cause of death was prostate cancer, and five controls for each case. Workers categorized in the highest 10 percent of EMF exposure were twice as likely to die from prostate cancer as those exposed to EMFs at lower levels, after adjustment for PCB exposure, race, and active work status within the past 2 years (odds ratio = 2.02, 95% confidence interval (CI): 1.34, 3.04). The odds ratio for PCB exposure and prostate cancer mortality was 1.47 (95% CI: 0.97, 2.24) after adjustment for suspected confounding factors. Exposure to high levels of both EMFs and PCBs showed no association with prostate cancer mortality. Non-White race was strongly associated with risk of prostate cancer mortality (odds ratio = 3.67, 95% CI: 2.66, 5.06). The association between EMF exposure and prostate cancer mortality warrants further investigation.