The authors studied the ultrastructural morphometry of secretory granules in four cases of thyroid medullary carcinoma. An inhomogeneous granules population was found with values ranging from 70 to 450 nm and mean values between 171.4 +/- 31.8 and 222.7 +/- 28.1 nm, respectively. Plurimodal frequency distribution curves showed peaks at 80, 90, 130, 160, 200, 240 and 250 nm. Two possibilities explaining the variation in granules size are discussed: the size of granules could depend on the maturation degree and functional activity or, more probably, there are several kinds of granules containing various biogenic substances, such as calcitonin and ACTH. The granules might play a role in the genesis of amyloid fibrils as well, some granules being located around amyloid fibrils.