The purpose of this work was to study the effects of high environmental exposure to polychlorinated biphenyls (PCBs) and other organochlorines on the thyroid. Thyroid volume, hypoechogenicity and nodules (by ultrasound), presence of antithyroid peroxidase (anti-TPO) antibodies, and abnormal thyroid-stimulating hormone (TSH) levels in serum (by radioimmunoassay) were examined in 101 adults from the PCB-polluted area in 360 controls. Serum levels of PCBs, hexachlorobenzene, γ-hexachlorocyclohexane (HCH), p,p′-DDT(1,1,1-trichloro-2,2′-bis(p-chlorophenyl)ethane), and p,p′-DDE(1,1-dichloro-2,2′-bis(p-chlorophenyl)ethene) were measured by high-resolution gas chromatography. Very high levels of PCBs were found in the polluted area (7300 ± 871 ng/g lipids) compared with controls (2045 ± 147 ng/g). Positive correlations (P < 0.001) were found between the levels of all organochlorines and their total except for hexachlorocyclohexane (HCH). In the polluted area, the highest thyroid volumes (18.7 ± 2.32 mL; mean ± SE) were clustered among 23 subjects (17 males and six females) with PCB levels above 10,000 ng/g (range 10,000–58,667 ng/g). In the remaining 438 subjects the thyroid volume was 14.2 ± 0.29 mL. These data suggest that there might be a threshold serum PCB level of approximately 10,000 ng/g that may influence the thyroid volume. A two-way ANOVA showed that all thyroid volumes in the polluted area were significantly higher (P < 0.001) than in the control area. In males from the polluted area, the frequencies of thyroid hypoechogenicity, thyroid nodules, positive anti-TPO, and abnormal TSH level were higher than in males from the control area, whereas such differences were not observed in females. Increased thyroid volume and indicators of potential thyroid dysfunction were associated with long-term environmental exposure to PCBs. These effects on the thyroid were confined to subjects with PCB levels above 10,000 ng/g of lipid (thyroid volume) and to males from the polluted area (thyroid hypoechogenicity, thyroid nodules, positive anti-TPO, and abnormal TSH).