Immunohistochemistry and immunofluorescence were performed on thyroid sections of 44 consecutive patients undergoing thyroid surgery for goiter due to iodine deficiency. Sections were compared with specimens from ten individuals without goiters from the same endemic area, with specimens from ten sporadic nontoxic goiter patients, and with specimens from an area with sufficient iodine supply from nine healthy subjects. Cells were characterized using monoclonal antibodies to the CR3 receptor (CD11b) and the p150/95 antigen (CD11c) present on macrophages, to HLA-DR, to antigen presenting cells (RFD1), to T helper (CD4) and to T suppressor/cytotoxic cells (CD8), and with a polyclonal antibody to human cytokeratin. In iodine deficient goiters, focal aggregates were found of RFD1-positive dendritic cells. Furthermore, RFD1-positive epitheloid cells were seen. In 27% of cases, these epitheloid cells completely filled the thyroid follicles. Within the epitheloid cell clusters, multinucleated giant cells could be detected that carried the macrophage markers. Dendritic cells, epitheloid cells, and giant cells were strongly HLA-DR positive. In nongoitrous thyroids from the endemic area such aggregates could also be seen but they were more sparse and were RFD1 negative. Giant cells were absent there. In normal thyroids with sufficient iodine supply, only a few isolated dendritic cells were seen. All except RFD1, which was negative, showed the same marker pattern. In sporadic nontoxic goiters from an area with sufficient iodine supply, dendritic cells occurred in much higher numbers than in the normal thyroids from that area, and they were RFD1 positive. They never aggregated as in iodine deficiency, and giant cells were not observed. These observations on iodine deficient goiter strongly suggest involvement of active antigen-presenting cells in this disorder. However, the immunohistologic difference between this disease and sporadic goiter suggests different underlying mechanisms.