A family was studied in which two members died from leukemia, two died from cancer, and two others had hereditary telangiectasia. Slight immunoglobulin abnormalities and cytogenetic mosaicisms were found among their apparently normal relatives. The neoplastic potential of these findings can only be inferred from their previous association with similar malignancies. Their occult nature is compatible with the dearth of information concerning susceptibility toward malignant diseases. An analysis of the literature indicates that a possible predisposition toward familial leukemia is directed toward malignancy in the broad sense rather than toward any specific variety. Useful information about leukemogenesis may be forthcoming from further similar studies.