A case of malacoplakia of the thyroid gland is described in a 50-year-old Japanese woman. This lesion clinically mimicked a malignant neoplasm, and the true diagnosis of malacoplakia was made only after histologic examination; light microscopy revealed a granulomatous nodule with an accumulation of von Hansemann's histiocytes containing PAS-positive and von Kossa's-positive intracytoplasmic and extracytoplasmic inclusions known as Michaelis-Gutmann bodies. There were some foci consisting of neoplasm-like or hyperplastic residual follicles within the lesion. Electron microscopically, a small number of bacilliform organisms were demonstrated within the lesion. X-ray microanalysis of Michaelis-Gutmann bodies was performed and revealed the presence of phosphorus, calcium, iron, and chloride. It is suggested that the malacoplakic lesion may be associated with the hyperplastic or neoplastic follicular lesion, and bacterial infection could be important in the causation of malacoplakia of the thyroid gland.