The lysosomal acid protease is thought to be a main enzyme in hydrolysis of thyroglobulin. The protease activity of lysosome-rich fractions of various diseased thyroid glands were assayed by the sensitive and reproducible method developed in our laboratory. They included 78 diseased thyroid glands; 37 thyrotoxic goitres, five irradiated thyroids, five Hashimoto's thyroiditis, one granulomatous thyroiditis, six multinodular goitres, 14 'cold' adenomas, four 'hot' adenomas with three paranodular tissues, and six carcinomas. Fifteen paranodular tissues of cold and solitary thyroid nodules were served as controls. The protease activities were significantly increased in thyroid tissues of thyrotoxic goitres, Hashimoto's thyroiditis, adenomas and adenocarcinomas. In thyrotoxic goitres, the protease activity correlated well with the mean area size of colloid follicles. The 'hot' adenomas had higher protease activity than 'cold' ones. The protease activity of multinodular goitres was nearly normal. Only the irradiated thyroid glands had low protease activities. Therefore the high protease activities in hyperfunctioning goitres seem to reflect hormone secretory activities. On the other hand, cold neoplastic goitres seem to have lysosomal protease activities which are not associated with thyroidal secretory process. The assay of acid protease activity will be useful to understand the pathophysiology of lysosomal system in diseased thyroid glands.