Short telomeres are a consistent feature of papillary thyroid cancer (PTC), which, in familial cases, is not restricted to tumor tissue, suggesting that familial PTC has a distinct, heritable, genetic background. Relative telomere length was compared between 30 patients with familial PTC and 46 patients with sporadic PTC. Mean relative telomere length in patients with familial PTC was shorter than that found in sporadic PTC tissues. In patients with sporadic but not with familial PTC, the mean relative telomere length in neoplastic thyroid tissue was significantly shorter than that found in normal contralateral tissues and extrathyroidal tissues.