Objective: To study the clinicopathologic features of diffuse large B-cell lymphoma (DLBCL) with expression of anaplastic lymphoma kinase (ALK) protein. Methods: Nine hundred and forty-five (945) cases of DLBCL (including 177 consultation cases) diagnosed according to the 2001 World Health Organization classification of tumors of hematopoietic and lymphoid tissues were enrolled into the study. Immunohistochemical study for anti-ALK-11 was performed using LSAB technique. The ALK-positive cases were further confirmed by immunohistochemical study using EnVision technique. Only ALK-positive cases by EnVision technique were further analyzed by immunostaining for antigens including CD20, CD3, CD30, EMA, granzyme-B, TIA-1 and PC. Immunoglobulin heavy chain gene rearrangement study was also performed and follow-up data collected. Results: There were altogether 5 (4 males and 1 female) cases of DLBCL showing expression of ALK protein. The age of the patients ranged from 34 to 72 years. All were primary nodal DLBCL. One case belonged to clinical stage I, 2 in stage II and 2 in stage III. The duration of follow up ranged from 4 to 32 months. Three patients subsequently died and the longest survival was 32 months. Morphologic subtypes included centroblastic 2, anaplastic 1, immunoblastic with plasmacytoid differentiation 1 and plasmablastic 1. Immunohistochemically, 4 cases were CD20 positive (including 2 centroblastic, 1 anaplastic and 1 immunoblastic cases). The plasmablastic case expressed kappa light chain and was negative for CD20. Rearrangement of immunoglobulin heavy chain gene was demonstrated in all 5 cases studied. As for ALK protein staining, a mixed membranous and cytoplasmic (1 immunoblastic case), granular cytoplasmic (2 centroblastic and 1 anaplastic cases) and mixed nuclear and cytoplasmic (1 plasmablastic case) patterns were observed. Conclusions: Expression of ALK protein is a rare phenomenon in DLBCL and can be seen in centroblastic, anaplastic, immunoblastic and plasmablastic subtypes. It is often associated with aggressive clinical behavior and worse prognosis. A new pattern of ALK protein expression, mixed membranous and cytoplasmic, is reported.