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This study describes three carcinomas and three benign follicle-cell lesions that consisted of vacuolated thyrocytes. In five of these tumors the neoplastic cells, wholly or in part, had the same unique signet-ringcell feature as recently described for a mucinous microfollicular thyroid adenoma case. On electron microscopy, however, intracellular lumina that were bordered by micro villi were shown. Serial sections of the epon-embedded material and the results of mucin stains and thyroglobulinimmunostaining excluded primary mucin-producing tumors of the thyroid and in each of our six thyroid lesions gave evidence of an arrest of folliculogenesis.