Background Approximately 8% of nonmedullary thyroid cancers are familial. The optimal age for screening in familial nonmedullary thyroid cancer (FNMTC) is unknown. Methods Kindreds with FNMTC (2 or more first-degree relatives affected) were screened prospectively with thyroid ultrasonography. Results Fifteen kindreds showed an overall prevalence of thyroid nodule(s) ≥5 mm of 44% at screening; 19% in the second generation, and 90% in the generation anterior to the index case. The youngest age of detection was 10 years for thyroid nodules and 18 years for thyroid cancer. Microcalcification of thyroid nodules at screening was associated with a greater risk of cancer ( P < .05). Family members diagnosed with thyroid cancer by ultrasonographic screening were diagnosed at a younger age and had a lower rate of extra thyroidal invasion ( P < .05). Conclusion In FNMTC, first-degree relatives 10 years or older, including the generation anterior to the index case, should have thyroid screening by ultrasonography, which may result in earlier diagnosis.