Anaplastic large-cell lymphoma (ALCL) is a recently proposed subset of non-Hodgkin's lymphoma. To determine whether Epstein-Barr virus (EBV) is associated with this lymphoma, we performed mRNA in situ hybridization on seven cases of ALCL using a probe consisting of an RNA sequence complementary to the transcripts of BamHIW fragment of the EBV genome. We detected BamHIW transcripts of EBV in the majority of atypical large cells of all cases of ALCL, but in none of three cases of lymphoblastic and small lymphocytic lymphomas. Furthermore, we detected latent membrane protein-1 (LMP1) in two cases of ALCL by means of immunofluorescence and immunoperoxidase stainings. These findings suggest that EBV is involved in the neoplastic transformation for ALCL as in the case of Hodgkin's disease, which shares several clinicopathologic features with ALCL.