Fifteen cases of chronic lymphocytic thyroiditis (CLT) diagnosed on the basis of both clinico-functional criteria and humoral autoimmunity (auto-ACAT in the serum) were studied. The auto-ACAT were avaluated both by Boyden's passive hemagglutination and by the complement fixation reactions. In all the diagnosis received histopathologic confirmation, the lesional picture being analysed from a qualitative-semiquantitative aspect. In 5 of the 15 cases, thyroid silver impregnation according to Slidders's technique was also done. Corroborating lesional aspects with humoral auto-immunity, we found a correlation between the extension of the inflammatory infiltrates dominated by lymphocytes and the titres of the antimicrosomal antibodies. The titres of the antithyroid antibodies do not correlate with the thyroid function status. Conversely, the extent of fibrosis agrees well with the incidence of myxedema. Silver impregnation reveals severe lesions of the argyrophilic fibres, with diffuse distribution. These lesional aspects may precede the real thyroiditic process.