Eleven allograft recipients (one cardiac, one hepatic, nine renal) at the Massachusetts General Hospital developed a lymphoproliferative disorder or leukemia. Six (all renal) patients received conventional immunosuppressive therapy (CIT), four received cyclosporin A (CsA) (one cardiac, one hepatic, two renal), and one received CIT for his first transplant and CsA for his second transplant (both renal). The interval from transplant to onset of the hematologic disorder ranged from 2 mo to 3 yr in the CsA group and from 6 mo to 9 yr in the CIT group and was 16 yr in the patient with two allografts. There were eight malignant lymphomas, seven of which were extranodal, (four immunoblastic, one large noncleaved cell, one small noncleaved cell, one plasmacytoma, one unclassifiable), one case of polymorphic diffuse B cell hyperplasia and two cases of acute myeloid leukemia. Frozen section immunohistochemistry in six cases showed monotypic immunoglobulin in four lymphomas, (including the plasmacytoma), an immunoglobulin-negative B cell phenotype in one lymphoma, and polytypic immunoglobulin in the case of polymorphic hyperplasia. One lymphoma showed a monotypic immunoglobulin-producing B cell population in one site and an immunoglobulin-negative B cell population in another site. With an immunoglobulin heavy chain gene-specific probe, Southern blot analysis of tissue from these two sites revealed two distinct rearrangements. When tissue from a second case of lymphoma was analyzed by Southern blot, identical rearrangements of the heavy chain gene were found in tumor from two separate sites. Similar to the experience of others, we find an increased incidence of lymphoma and a slightly increased incidence of acute myeloid leukemia in allograft recipients. In contrast to other reports, we found a predominance of monoclonal B cell malignancies, a more polymorphous histologic appearance of the lymphoproliferative disorders in CsA patients, and one case each of "multiclonal" and "monoclonal" lymphomas when tumor from separate sites was tested for gene rearrangement.