Advances in molecular biology techniques suggest that many haematological neoplasms originate from a transformation process at the level of the haemopoietic pluripotential stem cell. While familial aggregation has been reported for many haematological neoplasms, most studies were uncontrolled and examined the presence of the same haematological neoplasm as the index case. We assessed the familial aggregation of all haematological neoplasms in 4061 family members of 189 patients with various haematological neoplasms and two control groups: 955 relatives of 36 patients with non-malignant haematological disorders and 508 relatives of 33 patients with type II diabetes mellitus. Data collection included self-administered questionnaires. The odds ratio for haematological neoplasms among relatives of the index cases adjusted for age, sex, ethnicity, number of relatives in the family, and degree of familial linkage in the study group versus the two control groups was 3.62 (95% confidence interval, 1.44-9.07; P < 0.01). The vast majority of the haematological neoplasms among family members did not belong to the same histopathological category as the index cases. The data support the hypothesis of a genetic predisposition to haematological neoplasms. The fact that the aggregation is not disease specific is consistent with a defect in the pluripotent haemopoietic stem cell.