OBJECTIVE: To quantify the yearly prevalence of Hashimoto's thyroiditis at our Division and evaluate changes in its clinical presentation over the period 1975-2005. DESIGN: We have reviewed retrospectively the clinical records of patients admitted to a university hospital located in Messina, Sicily (Italy), from January 1, 1975, through December 31, 2005. This facility attracts patients from North-Eastern Sicily and most of Calabria, the Italian region across the Straits. HT was diagnosed based on various combinations of clinical, laboratory, and instrumental findings. The study group consisted of 4064 HT patients diagnosed during these 31 years. MAIN OUTCOME: We evaluated these indexes on a yearly basis: number of new HT diagnoses; age at presentation; male-to-female ratio; functional status; goitrous or nongoitrous variants with or without nodule(s); above-normal titers or levels of serum thyroid autoantibodies (thyroglobulin antibodies and thyroperoxidase antibodies). Several indexes have changed over those 31 years. Particularly, HT has become 10 times more common than it was until the early 1990s and males are relatively more represented. Patients are relatively younger, with a nongoitrous thyroid; the rate of S-Hypo exceeds largely the rate of O-Hypo. Serum thyroid autoantibodies have become less frequently positive, and when positive, they are present at a lower concentration. CONCLUSIONS: Only environmental changes, as opposed to genetic changes, can account for such alterations in the presentation of HT in our geographical area.