Chronic lymphocytic leukemia (CLL) is a chronic lymphoproliferative desorder (CLPD) with peculiar epidemiologic characteristics. It is a disease of the elderly, which is very rare in under 30-year-old individuals and absent among children. Its incidence largely varies according to the ethnical origin: CLL is the most common leukemia in Western countries while it is rarely seen in Eastern countries. The etiology of CLL is still unknown. Environmental factors such as exposure to ionizing radiation (atomic bomb survivors did not show an increased incidence of CLL) or toxic or viral agents are not associated to the occurrence of CLL. Familiar CLL is characterized when at least two members of the same family develop CLL The diagnosis of familiar CLL seems to be made 10 to 20 years earlier than sporadic cases. CLL occurs more frequently among siblings and offspring although it may appear in other members such as cousins. The frequency of other CLPD (relative risk is 30 times greater) and monoclonal CD5+B cells proliferations (13.5%-18%) are higher among healthy members of familial CLL and HLADR1/11 is implicated in at least some populations with familial CLL.