During the course of our consultation activity, we have recognized a peculiar form of thyroiditis in which multiple foci of fibrosis, most of which were associated with reactive atypia of the surrounding follicles, are present. We have referred to this condition, both in our consultation reports and in the third series of A.F.I.P. Fascicle on Tumors of the Thyroid Gland, as "multifocal fibrosing thyroiditis" or (less frequently) "multifocal sclerosing thyroiditis," which are descriptive terms that highlight the benign/inflammatory nature of the process, its multiplicity, and its unknown pathogenesis. The aim of this study is to better define the morphologic features of this process and correlate it with some clinical data. With this purpose, the consultation files of one of the authors (J.R.) were searched for cases coded as multifocal fibrosing thyroiditis or multifocal sclerosing thyroiditis in a 20-year period ranging from January 1989 to December 2009. A total of 55 cases were identified that displayed the above-listed features. There were 51 (93%) female and 4 (7%) male patients (F/M=12.75), with ages ranging between 15 and 71 years (mean age, 47.03 y; median age, 44.5 y). Microscopically, multiple foci of fibrosis were identified in all cases, their number ranging from 2 to 51 per case (mean number, 16), with a mean diameter of 3 mm (range: 0.36 to 15.1 mm). Although heterogenous in shape and size, the individual foci were rather similar to each other in composition, being characterized by a fibrotic poorly cellular center that merged with a cellular peripheral zone. Some of the follicular structures present at the periphery of the scar and-to a lesser extent-those entrapped inside it underwent complex reactive and regenerative (atypical) changes that simulated malignancy. We discuss the differential diagnosis with other benign and malignant thyroid conditions and speculate about its pathogenesis and possible relationship with papillary thyroid microcarcinoma.