The nuclei of 17 occult and common papillary thyroid carcinomas were studied with EM. In most of the nuclei, numerous clusters of large interchromatin granules were seen. Nucleoli with a microfibrillar cortex and segregation of their components were observed in some tumors. Dense RNA containing microspherules were found in altered nucleoli and in complex nuclear bodies. These ultrastructural features seem to reinforce the advanced involvement of a disturbed RNA metabolism in human papillary thyroid carcinomas.