The association between thyroid cancer and familial adenomatous polyposis (FAP), albeit rare, is well known. It has been suggested that the thyroid tumors have unique histologic characteristics and may be follicular in origin. Because of their rarity, treatment and long-term prognosis are uncertain. Twelve such patients (prevalence 399/100,000) seen during 1949–1995 were retrospectively reviewed. Histology was independently re-reviewed by two pathologists. There were 11 female patients (two sisters) and 1 male patient, with a mean age of 28 years (range 15–61 years). Eight patients (66%) had multicentric tumors and five (42%) bilateral disease. Average tumor diameter was 1.8 cm (range 0.2–5.0 cm). Regional nodal metastases were present in two patients. All 12 thyroid cancers in this series were papillary. The one male patient demonstrated “typical” histology with variable papillary and follicular architecture, whereas the 11 female patients had tumors with unusual histology as described by Harach. Five patients (41%) were treated by total thyroidectomy, five with near-total thyroidectomy, and two with lobectomy alone. Mean follow-up was 142 months (range 7 months to 30 years). Regional recurrent disease occurred in two patients, one of whom died of the disease. The 5- and 20-year survivals were 90% and 77%, respectively. The results indicated that all tumors in this study were papillary, although atypical histology was encountered in 91%. The mean age (28 years) is younger than that of patients with sporadic disease. Multicentricity and bilateral disease are common. In view of this finding, total thyroidectomy should be strongly considered. Long-term prognosis is excellent. The finding of unusual histology in a young patient with papillary thyroid carcinoma should arouse the suspicion of FAP.