True histiocytic lymphoma (THL) is a very rare type of non-Hodgkin's lymphoma (NHL) in which neoplastic cells exhibit markers of histiocytic differentiation. Some cases of THL have been reported in patients with previous acute lymphoblastic leukaemia (ALL), especially in children and young adults, in whom the acute leukaemia was of T-cell origin. The relationship between the initial lymphoid tumour and the secondary THL remains unclear, as a common monoclonal origin shared by both neoplasms has never been definitively demonstrated. We report a patient with B-ALL who developed a nodal and extranodal tumour with histological and immunohistochemical features of THL 4 years after the initial diagnosis. Genotypic study showed that both neoplasms contained the same immunoglobulin heavy gene rearrangement, which has not been reported previously.