A case of posttransplantation lymphoproliferative disorder (PTLD) involving the pleura is reported. The patient was a 57-year-old man who underwent liver transplantation 2 years prior to the development of PTLD. The PTLD was pleural-based and was first detected by radiologic studies as a pleural effusion. Transbronchial biopsy and cytologic examination of 2 pleural fluid specimens were nondiagnostic. Subsequent open-wedge biopsy revealed a monomorphic PTLD, composed of large immunoblasts with plasmacytoid differentiation. Immunohistochemical studies demonstrated B-cell lineage with expression of monotypic cytoplasmic immunoglobulin kappa light chain and CD79a, and absence of T-cell antigens. Immunohistochemical and in situ hybridization studies demonstrated Epstein-Barr virus protein and RNA, respectively. No evidence of human herpesvirus 8 DNA was detected by polymerase chain reaction. We report this case because pleural-based PTLD is rare. The diagnosis of this entity is made more difficult by the fact that PTLD is often underrepresented in pleural fluid cytology samples.