Fine-needle aspiration biopsy (FNAB) has been used with high sensitivity and specificity in the diagnosis of both Hodgkin's and non-Hodgkin's lymphoma. However, studies of FNAB of posttransplant lymphoproliferative disorders (PTLDs) are rare. The clinical course of 593 allograft recipients (cardiac, 288; renal, 250; lung, 50; and heart/lung, 5) was reviewed. Twenty-six patients developed PTLD with an overall incidence of 4.4%. Of these patients, 12 underwent FNAB. Their age ranged from 33-67 yr (mean, 55 yr). The interval between transplantation and FNAB ranged between 2-14 mo (average, 8.4 mo). The lungs were the most common site aspirated (7 cases), followed by lymph nodes (3 cases) and other extranodal sites (2 cases, liver and paraspinal mass). The cytologic features of these aspirates could be classified into two categories: a polymorphous smear composed of a spectrum of mature and immature lymphocytes with scattered plasma cells and histiocytes; and a monotonous population of large lymphoid cells consistent with malignant lymphoma, large-cell type. Surgical biopsies were available in 10 (83.3%) cases and confirmed the FNAB diagnosis. In summary, FNAB appears to be a highly sensitive and specific diagnostic tool in patients with PTLD.