Mass screening for the purpose of detecting thyroid cancer at its earliest stage may not be recommended at the present time, but screening focused on certain risk groups is advocated. Familial nonmedullary thyroid carcinoma (FNMTC) is a distinct clinical entity with a high incidence of multifocality and association with multiple benign nodules. FNMTC patients have shorter disease-free survival than do sporadic disease patients because of frequent locoregional recurrence. Screening by neck ultrasonography was performed for symptom-free family members of patients with FNMTC. A total of 149 subjects representing 53 FNMTC families were examined in this study. The average age of the patients was 41.0+/-19.0 years (range 3-76 years) with a female/male ratio of 104:45. At least one thyroid nodule was found in 77 (51.7%) of the 149 patients. Surgery was performed in 18 patients, and thyroid cancer was discovered in 15 of them (10.1%; 14 women, 1 man; 32-61 years of age). Papillary thyroid cancer (PTC) was found in 14 and follicular thyroid cancer combined with PTC in 1. The tumors averaged 9.1+/-5.4 mm (3-21 mm) in greatest diameter. Intraglandular metastases were found in 7 (47%) of the 15 patients with thyroid cancer. Lymph node metastases were found in 6 (43%). Ultrasonographic screening for FNMTC family members may enable the discovery of asymptomatic thyroid cancer. Because of the high incidence of intraglandular and lymph node metastases, we recommend screening with ultrasonography for early detection in family members of patients with FNMTC.