This study describes nine cases of post-transplant lymphoproliferative disease (PTLD) presenting as renal allograft dysfunction. Onset of symptoms was 34 to 265 days post-transplant, typically (in six of nine cases) after refractory rejection treated with OKT3. Diagnosis was made by histopathologic examination of needle biopsy (three of nine cases) or allograft nephrectomy (six of nine cases) specimens. Disease was confined to the allograft in three patients. The morphology was polymorphic in eight cases and monomorphic in one case. Five cases showed monotypic kappa or lambda light chain expression. Expansile lymphoid infiltrates, serpiginous necrosis, nuclear atypia, and presence of Epstein-Barr virus RNA helped to distinguish PTLD from severe rejection. Tubular damage and venulitis was common in PTLD lesions, but arterial involvement was not prominent. Infiltration of the ureter, hilar adipose tissue, and nerve twigs was frequent in nephrectomy specimens. Reduction of immunosuppression led to resolution of PTLD in two of three cases diagnosed by needle biopsy, but severe acute rejection led to graft loss in one case; the third case progressed to fatal multisystem disease. Among cases diagnosed at nephrectomy, two of six patients died of disseminated PTLD and one of six died of sepsis. The five surviving patients are alive 41 to 99 months after initial diagnosis without evidence of recurrent PTLD.