Intranuclear cytoplasmic inclusions (ICI) in 38 cases of thyroid papillary carcinomas have been studied as to the relationship between the frequency of ICI and the clinical, histological, and cytological findings of the carcinoma. ICI were present in all cases. The frequency of ICI in the cases studied ranged from +/- (one per several fields) to ¿ (more than ten per one field), using a microscope of 400 magnifications, and from 1.3% to 11.6% (mean value: 3.9%) of the nuclei of the carcinomas. The frequency of ICI was found to be influenced by the state of the nuclear chromatin and pleomorphism, but was not influenced by the papillary or follicular tumor growth. The relationship between the frequency of ICI and the tumor size and the existence of lymph-nodes metastasis, however, was not clarified.