Cells belonging to the mononuclear phagocytic system may give rise to a wide variety of proliferative disorders. These neoplastic or reactive diseases of monocytes, histiocytes and macrophages are still poorly understood, both from a biological, clinical and pathological point of view. In 1984 a new type of non-Hodgkin's lymphomas has been described, which recently was termed large cell anaplastic lymphoma (LCAL). Since both "true" malignant histiocytosis (MH) and LCAL display some clinico-pathological similarities, we reevaluated eleven children with the primary diagnosis of MH, who entered the multicentric therapy study DAL-HX 83. On the basis of the typical morphology LCAL and not MH has been diagnosed in all cases. The Ki-1 (CD 30) positive tumor cells of LCAL can display different phenotypes, either T-lymphoid, B-lymphoid, histiocytic or neither of them (O-phenotype). Therefore we investigated the lineage specificity of tumor cells in 15 paediatric patients with LCAL. Tumor cells of histiocytic origin could be demonstrated in only one of them. In 7 patients the neoplastic cells were of T cell and in the remaining 7 cases of non T/non B cell (O) phenotypes, confirming the extreme rarity of MH in children. It is our opinion that MH and LCAL seem to be different diseases not only nosologically but also clinico-pathologically. In the multicentric therapy studies NHL-BFM 81 and NHL-BFM 83 the frequency of LCAL in childhood is compared to all other subentities at 3%.