Introduction In the general literature on the etiology of malignant neoplasms the rôle of heredity has had comparatively little place, and only in late years has serious attention been paid to this question. Recent research on the heredity of malignant neoplasms has included genetic investigations on animals (chiefly mice) and genetic studies of human neoplasms. As regards the first type of investigation, although the majority of authors agree as to the importance of hereditary factors, their conclusions are conflicting; the mode of inheritance, the number of involved genes, and their relation to the environment are still open questions. Still less clear is the rôle of heredity in the etiology of human neoplasms. The late onset of the disease, its high incidence and varied localization, and the variety of tumor types combine to make the problem extremely complicated. Data on the genetics of malignant neoplasms in man have been of two kinds: (1) isolated case histories, constituting the majority of reported investigations, and (2) systematic studies. Among the former may be included reports on malignancy in twins (Burkard; McFarland and Meade; Militzer; Champlin; Benedict; Croom, and others). Selected material of this sort, however, is of little if any value in serious genetic analysis, as Levit has shown. Systematic investigations have been few in number (Miche, Hunter, Wassink, Sandberg and Goryainova, Kranz, Waaler). Of these Waaler's stands out both for quality and quantity of material. The findings in human cancer, like the experimental findings, are conflicting. The reason for this may be sought both in the complexity of the problem and in the shortcomings of the investigative methods, not the least of which is an attempt to study the problem as a whole, without differentiating the character and localization of the disease.