Riedel's thyroiditis is a rare disease which has in the past been confused with the much commoner fibrous variant of Hashimoto's thyroiditis. We have compared the histological and immunohistochemical characteristics of three cases diagnosed as Riedel's thyroiditis with five cases of the fibrous variant of Hashimoto's thyroiditis. The major distinguishing features on light microscopic study were that Riedel's thyroiditis showed (a) extra-thyroid extension of the fibrous tissue, (b) a phlebitis with distension of the vein lumen by lymphoid or fibrous tissue, and (c) relatively normal surviving thyroid tissue, while in Hashimoto's thyroiditis the fibrous tissue did not extend outside the thyroid, the veins were surrounded by fibrous tissue, but did not show a phlebitis, and all thyroid tissue was affected by the process. These findings confirm those of other workers in suggesting that there is a clear histological distinction between these two processes. The quantitative immunohistochemical studies showed that in the fibrous variant of Hashimoto's thyroiditis, cells containing kappa chains outnumbered lambda chain cells in all cases, the mean ratio of kappa to lambda being very close to that found generally in the circulation, with lambda chain cells forming 36% of all light chain containing cells. In contrast, the mean proportion of lambda chain cells in Riedel's thyroiditis was 71%. In Hashimoto's thyroiditis the dominant plasma cell was, as expected, the IgG containing cell, with IgA cells forming only 15% of all heavy chain cells. In Riedel's thyroiditis IgA containing plasma cells were unusually prominent, forming 47% of all plasma cells present. These findings confirm the separation of the two entities, and demonstrate an unusual pattern of restriction of antibody forming cells in Riedel's thyroiditis. It is possible that IgA plays a role in the pathogenesis of Riedel's thyroiditis.