Almost half of all lymphomas arise at extranodal sites where they often perplex general surgical pathologists as well as hematopathologists. Beginning with a brief review of non-Hodgkin's lymphoma classification including the principles and general format of the new WHO classification, this course focuses on the specific extranodal lymphomas now considered as distinct clinicopathologic entities. Case presentations will be used to describe the initial workup of these diseases, illustrate the diagnostic criteria, and discuss the reactive and neoplastic processes included in the differential diagnosis. The histopathologic, immunophenotypic and genotypic features of each entity will be presented along with the clinical implications of each diagnosis. The use of paraffin section immunohistochemistry will be emphasized including its use in the differential diagnosis of small B-cell lymphoid neoplasms and its role in defining T cells and natural killer (NK) cells and in the diagnosis of T/NK cell neoplasms. The B-cell proliferations that will be discussed include gastric marginal zone B-cell lymphoma of mucosa-associated lymphoid tissue (MALT) type, mantle cell lymphoma involving the gastrointestinal tract (multiple lymphomatous polyposis), salivary gland lymphoid lesions and splenic marginal zone lymphoma. The T/NK-cell proliferations to be discussed include nasal NK/T cell lymphoma, hepatosplenic gamma-delta T cell lymphoma, primary cutaneous anaplastic large cell lymphoma (ALCL), and subcutaneous panniculitis-like T-cell lymphoma. This course is intended for trainees in anatomic pathology or hematopathology and for general surgical pathologists. A limited number of loan sets of glass slides will be available for advance mailing. All registrants for the course will be mailed a set of transparencies after the meeting. A syllabus will be distributed at the course. (Last Scheduled Presentation)