A review of the literature will show that there is little evidence of genetic factors in the aetiology of the great majority of cases of leukaemia. Nevertheless it would appear that in some families characterized by multiple leukaemia, heredity may be significant in determining susceptibility to the diseases. This applies especially to chronic lymphocytic leukaemia which, it would appear, occurs in the majority of cases of familial leukaemia, Also, the high incidences of other independent malignancies in patients with this form of leukaemia, and the possibly raised incidence of cancer in families of these patients, are suggestive of a genetic constitution conferring increased susceptibility to chronic lymphocytic leukaemia and the malignancy in general. This view was supported by the finding of an inherited chromosome abnormality in a family with two cases of chronic lymphocytic leukaemia in sibs (Fitzgerald et al., 1962; Gunz et al., 1962) a more complete account of which is presented in this thesis. This finding in turn, prompted a more complete investigation of instances of multiple leukaemia in families.