Multiple thyroid involvement (MTI) in papillary thyroid carcinoma was clinicopathologically studied in 105 non‐selected, consecutive patients. Whole thyroids resected by total thyroidectomy were sectioned at intervals of 2–3 mm (mean number of slices per gland, 19.2) and histologically reviewed. The intraglandular cancer foci, other than the tumor regarded as the primary focus, were demonstrated in 82 (78.1%) of 105 patients. The foci were usually small (less than 4 mm) and were not accompanied by sclerotic fibrous stroma or by a fibrous capsule. These small foci were distributed around the primary lesion and also were found frequently (61.0%) in the opposite lobe as bilateral disease. In the opposite lobe, a similar incidence (approximately 30%) of disease was obtained in each of the three parts (upper, middle, and lower). The mean number of foci in patients with MTI was statistically correlated to age, the presence of lymph node metastases, and the presence of solid areas or psammoma bodies in the primary tumor. It was concluded that MTI could be regarded as one of the most striking and important biologic characteristics of papillary thyroid carcinoma.