Two familial cases of hairy cell leukaemia are reported: a daughter, 44-years-old, with a very unusual ultrastructural pattern found in hairy cells, the "tubuloreticular inclusions", and her mother, 71-years-old, who was affected six years later. Routine laboratory investigations, cytochemical and cytogenetic studies including HLA typing, as well as in vitro proliferative response of peripheral blood mononuclear cells (PBMC) to polyclonal mitogens and to exogenous interleukin 2, were performed. The immunological characterization by assessing the cell surface phenotypic markers with monoclonal antibodies and transmission electron microscopy (TEM) investigations were also carried out. In case 2 all tests were performed both on PBMC and on the bone marrow cells. To the best of our knowledge this is one of the first reports of such familial association. The possibility that genetic factors might play a role in the etiology of leukaemia in man is discussed: in our two cases, however, cytogenetic studies did not support this, while HLA typing revealed a non-significant association of HCL with DQw3 allele. Alternatively, an environmental factor has been considered, and a viral infection-perhaps by a retrovirus of the HTLV family has been suggested as tubuloreticular inclusions have been found in both hairy cell leukaemia, as reported, by us, and AIDS-LAS. However, a long time elapsed between the manifestation of HCL in the daughter and in the mother, and as the two patients had not been living together at that time, the possibility of a viral transmission seems minimal. The results of TEM and of immunological investigations are presented and discussed. Both, but particularly the latter, support the B cell nature of the hairy cell.