A previously characterized cohort of 115 Great Lakes fisheaters and 95 non-fisheating controls was re-examined in 1989 to evaluate changes that had occurred in serum PCB and DDT levels since the 1982 study. Substantial and significant decreases in mean serum DDT levels had occurred in both fisheaters (25.8 ppb vs 15.6 ppb) and controls (9.6 ppb vs 6.8 ppb) over this time period. In contrast, only a slight decrease in serum PCB levels was observed, and in fisheaters only. No association between individual changes in serum PCB or DDT levels and self-reported changes in Great Lakes fish consumption was observed. The findings from this longitudinal examination of serum PCB and DDT levels confirm earlier cross-sectional surveys of western populations, and demonstrate that the prohibition of DDT has been successful in reducing the level of DDT contamination in human populations.