A longitudinal human biomonitoring study has been performed in two Hungarian primary aluminium production plants that operated Söderberg cells. Carcinogen-DNA adducts have been determined by 32P-postlabelling and competitive enzyme-linked immunosorbent assay in peripheral blood lymphocytes from potroom workers and occupationally unexposed control individuals. Blood samples were collected on three occasions; the first two occasions were 1 year apart during normal operation, and the last samples were taken 6 months after close-down of aluminium production. Assays of the first set of samples demonstrated no significant difference between the control group and workers in Plant I. Workers in Plant II had significantly higher DNA adduct levels than individuals in the control group and workers in Plant I. One year later a significant elevation of DNA adducts was detected in Plant I so that values approached those seen in Plant II, which remained unchanged. In the last sample set there was no difference between former potroom workers and occupationally unexposed individuals. The results suggest that carcinogen-DNA adducts are a useful biomarker for monitoring occupational genotoxic exposure to polycyclic aromatic hydrocarbons, and that the findings can contribute to improved health risk assessment.