Several pre-malignant diseases are known to have a genetic etiology. This study focuses attention upon precancerous disorders wherein the mode of inheritance is either well established or wherein it remains unclear even though familial aggregation of the particular diseases have been amply documented. These conditions will be discussed as useful models for systematic investigations of the host etiologic component in carcinogenesis. Our survey of hereditary precancerous syndromes includes multiple polyposis of the coli, the multiple mucosal neuroma syndrome, the Cancer Family Syndrome, Sipple's syndrome, Von Recklinghausen's neurofibromatosus, the multiple nevoid basal cell carcinoma syndrome, tuberous sclerosis, familial cutaneous malignant melanoma, and carcinoma of the breast. We have emphasized the heterogeneous character of many forms of familial cancer. Familial breast cancer associations clearly show such heterogeneity, as do colon cancer syndromes. Certain of these precancerous states are characterized by phenotypes which are clinically apparent, polyposis coli being the classic example. Others, such as Sipple's syndrome are amenable to routine screening for biochemical markers. The bulk of putative genetic cancer-predisposing problems require further basic investigation of modes of inheritance. Cancer control may be enhanced through communication of useful genetic and diagnostic information to primary care physicians. Referral of cancer clusters of possible genetic etiology from clinicians to human geneticists facilitates the necessary basic research.