The t(2;5)(p23;q35) translocation is associated with a high percentage of anaplastic large-cell lymphomas (ALCL) of T- or null-cell phenotype. This translocation was recently cloned and results in the fusion of the nucleophosmin gene (NPM) on chromosome 5q35 to a novel tyrosine kinase-encoding gene designated anaplastic lymphoma kinase (ALK) on chromosome 2p23. Using a sensitive and specific reverse transcription-polymerase chain reaction (RT-PCR) assay to detect the NPM/ALK fusion transcript, we assessed the involvement of NPM/ALK in a series of histologically and immunohistochemically confirmed ALCL, in non-ALCL aggressive non-Hodgkin's lymphomas of T-cell phenotype, and in Hodgkin's disease (HD) to better define the morphologic spectrum of disease associated with this translocation. Twenty-four cases of ALCL were selected on the basis of CD30 positivity and histologic features. Seventeen cases presented as classical nodal and extranodal disease, four cases presented as primary cutaneous disease, and three were associated with human immunodeficiency virus (HIV) infection. As ALCL may show overlapping histology with both HD and other aggressive non-Hodgkin's lymphomas, particularly of T-cell phenotype (T-NHL), we also studied 34 cases of HD and 19 of T-NHL. NPM/ALK chimeric transcripts of identical size were detected in 11 of the 24 (46%) cases of ALCL. NPM/ALK fusion transcripts were found in 11 of 17 (65%) classical ALCL cases but were not detected in the four primary cutaneous cases of ALCL or in the three HIV-related ALCL cases. In addition, NPM/ALK transcripts were not detected in any of the 34 cases of HD or in the 19 cases of T-NHL. These data indicate that NPM/ALK fusion transcripts occur in a high percentage of classical nodal ALCL (65%). In addition, these data strongly suggest that ALCL, as defined in this study, is not pathogenetically related to either HD disease or the majority of other types of aggressive T-NHL. This is a US government work. There are no restrictions on its use.