Background: The only known risk factor for chronic lymphocytic leukaemia (CLL) is occurrence of the disease in close relatives. The aim of this study was to determine the frequency of familial chronic lymphocytic leukaemia.
Material and method: All patients with chronic lymphocytic leukaemia notified to the Cancer Registry in the period 1.10.2007-31.12.2009 were asked to report occurrences of malignant disease in siblings, parents, grandparents and children. The information about malignant haematological disease was verified with the Cancer Registry.
Results: We found malignant haematological disease in close relatives of 42 of the 236 included patients (18%). CLL and lymphoma were the most common diagnoses. On average, 16 family members were identified in each family. The relative risk of developing CLL was six times higher in those who had close relatives with the disease (16 of a total of 3,776 family members) than among those who did not have close relatives who were affected (76 cases among 107,223 family members of 38,159 control subjects). The increased risk of disease was also associated with other lymphoproliferative diseases. With patrilinear, but not matrilinear inheritance, we found a birth order effect, with affection of younger men in a group of siblings, while the eldest escaped.
Interpretation: Malignant haematological disease is common in the family members of patients with CLL. CLL is the most common disease, but there is extensive pleiotropy.