A T-cell neoplasia showing clinicopathologic features of malignant histiocytosis with novel chromosomal abnormalities and N-ras mutation. Malignant histiocytosis (MH) is a distinct disease entity defined clinically and morphologically. However, the neoplastic origin of MH is not well established. The authors report a 26-year-old woman who showed the typical clinicopathologic features of so-called MH. Cytogenetic and molecular genetic examinations were performed in addition to the morphologic and immunologic approach. The expression of CD2 and T-cell receptor gene rearrangements indicated the T-cell origin of this case. CD30, which is positive for anaplastic large cell lymphoma (Ki-1 lymphoma), was not expressed. The cytogenetic study revealed a clonal chromosome abnormality involving 3q25, 6p21, 11p15, and 11q21. An N-ras point mutation within codon 12 (GGT----GCT) was also detected. These finding indicate that MH defined clinically and morphologically is not a tumor of true histiocytic origin and that it should be reclassified on the basis of immunologic, cytogenetic, and molecular genetic data. 