Dyshormonogenetic goiters (DG) are genetically determined thyroid hyperplasias due to enzyme defects in thyroid-hormone synthesis. We report 56 cases of DG occurring in 34 females and 22 males. The patients age ranged from newborn to 52 yr (median 16), 75% of the cases occurring before the age of 24. All patients presented with clinically evidence of goiter except for two patients that were diagnosed at autopsy. Hypothyroidism was documented before the histological diagnosis was made in 36 patients (64%). The thyroid gland was enlarged and multinodular in all cases, weighing up to 600g. Microscopically, the most common alteration consisted of markedly cellular nodules exhibiting a variety of architectural appearances, the solid and/or microfollicular patterns predominating. Papillary proliferations and an insular growth pattern were also present. Fibrosis was a common finding; in some instances it was very conspicuous, resulting in irregularities at the edge of the nodules simulating capsular invasion. Other constant features included marked nuclear atypia and minimal amount of colloid. In 18% of the cases, the degree of hyperplasia and atypia were such as to result in a mistaken diagnosis of follicular, papillary, medullary, or undifferentiated carcinoma. Three of the glands contained incidental small neoplasms fulfilling the criteria of papillary microcarcinoma; one of them was multicentric. The presence in a thyroid gland of the combination of these morphologic features should suggest the diagnosis of dyshormonogenetic goiter. The only other condition we are aware of that can result in a similar microscopic picture is iatrogenic goiter resulting from the administration of antithyroidal agents.