This study was conducted to clarify the actual morbidity and clinicopathologic features of thyroid neoplasia in patients with familial adenomatous polyposis. We analyzed the clinical records of 49 Japanese patients with familial adenomatous polyposis diagnosed at our institute. Six of these patients had thyroid neoplasias (3 carcinomas, 2 adenomas, and 1 of unknown histology). Among 17 patients (11 men and 6 women) who had been examined by thyroid ultrasonography or in whom postmortem examination was performed; we categorized 6 patients with thyroid tumor as the TT group, and the remaining 11 without thyroid tumor as the NTT group. The clinical features of the two groups were compared. Thyroid neoplasia was detected by physical examination in 2.3% of 43 patients so examined and was detected in 25% of 8 patients examined by thyroid ultrasonography, and in 44% of the 9 patients subjected to postmortem examination. There were no significant differences between the TT and NTT groups in gender; age at first admission, colectomy, and last follow-up visit; number of colonic polyps; or in presence of colorectal cancers, gastroduodenal adenomas, gastric fundic gland polyposis, retinal pigmented lesions, or extraintestinal tumors. The thyroid gland is frequently affected in patients with familial adenomatous polyposis, but there may be no association between thyroid neoplasia and other clinical manifestations.