We experienced four biopsy cases of medullary carcinoma of the thyroid gland and brief clinical courses and detailed histological findings have been reported. Three cases were associated with amyloid. All cases were positive for the argyrophil and negative for the argentaffin staining. Demonstaration of the argyrophilic granules in the tumor cells and stromal amyloid is a convincing finding to make a diagnosis of medullary carcinoma of the thyroid.