Chronic lymphocytic leukaemia (CLL) is a monoclonal proliferation usually involving B cells and composed of mature lymphoid cells. Distinct morphologic subtypes have been recognized according to lymphocyte size, nuclear:cytoplastic ratio and nucleolus. However the presence of characteristically binucleated lymphocytes in patients fulfilling criteria for CLL diagnosis has never been described. We here report immunological and cytogenetic studies of four patients with CLL but with binucleated lymphocytes. Moreover, trisomy 12, known to be associated with atypical morphology in CLL, was detected in two of these four patients. We suggest that this be considered as a possible new entity.