Carriers of cancer genes and certain members of cancer families can have a nearly 100% risk of developing specific cancers, often at unusually early ages. In some of these patients, precursor states can identify genetic predisposition to cancer and lead to appropriate measures for disease prevention and early detection. Precursors can be premalignant lesions, such as dysplastic nevi that transform to malignant melanoma, or associated disorders, such as congenital aniridia followed by Wilms tumor. Laboratory markers can also identify a precursor state for cancer, eg, elevated calcitonin levels in patients with familial medullary thyroid carcinoma and premalignant thyroid C-cell hyperplasia. The identification of carriers of cancer genes provides rare opportunities for life-saving intervention and for studies of the pathogenesis of human cancers.