Riedel's struma is one among the locations of an invasive fibroinflammatory and sometimes multifocal process, of unknown etiology. This disease is extremely rare. The authors report a further case in a 41 year old man who had been developing a woody goiter for 3 months. Laboratory investigations showed normal inflammation tests, high rates of antithyroid antibodies and mild hypothyroidism. A surgical operation only consisted in a biopsy, which revealed typical histological features. The only treatment used was a suppletive therapy. During the evolution, antithyroid antibodies progressively decreased, hypothyroidism moderately increased and, chiefly, the goiter completely disappeared after 18 months. To our knowledge, regressive evolution was reported in very few cases after corticoid therapy, but never spontaneously.