Thyroids from 213 adult herring gulls of both sexes were collected during incubation from nine colonies in the Great Lakes basin of eastern North America between 1974 and 1983, and from a single colony in the Bay of Fundy from 1977 to 1982. Qualitative and quantitative histological assessment revealed that the majority of the gulls from the Great Lakes basin suffered from goiter. These thyroids had a greater mass than those from the Bay of Fundy, and were microfollicular and frequently hyperplastic. The histopathology was similar to that previously observed in Pacific salmon from the Great Lakes. These findings are consistent with a forage fish-borne goitrogenic etiology other than, or in addition to, iodine deficiency. Temporal and spatial differences in the severity of thyroid dysfunction are consistent with the hypothesis that polyhalogenated hydrocarbons are responsible for the goiter development and thyrotoxic effects observed in herring gulls from the Great Lakes area.