Although the responsible gene has not yet been identified, patients with differentiated thyroid carcinoma, including papillary and follicular carcinomas, demonstrating a family history have been reported and patients having one or more family members with differentiated carcinoma among their first-degree relatives are designated as having familial nonmedullary thyroid carcinoma (FNMTC). In this study, we investigated the biological characteristics, including prognosis, of familial follicular carcinoma. Three hundred and nineteen patients who underwent initial surgery for follicular thyroid carcinoma between 1987 and 2004 who were enrolled in this study. Of these 319 patients, 6 patients (1.9%) in 6 families were classified as having familial follicular carcinoma based on the criteria described above. The incidence of aggressive characteristics such as male gender, age 45 years or older, poor differentiation, widely invasive carcinoma, tumor larger than 4 cm and distant metastasis at diagnosis did not differ between familial and sporadic follicular carcinomas. One patient with familial follicular carcinoma underwent re-operation because of newly detected papillary carcinoma in the remnant thyroid 160 months after the initial surgery, but none of the 6 patients with familial carcinoma showed recurrence or died of follicular carcinoma. We can therefore conclude that FMNTC of the follicular type is very rare and there is no evidence that familial follicular carcinoma is more aggressive or has a worse prognosis than sporadic follicular carcinoma. The therapeutic strategy for follicular carcinoma might depend on conventional prognostic factors such as poor differentiation and distant metastasis at diagnosis, but not on whether the carcinoma is familial or sporadic.