Dual rearrangement of both immunoglobulin and T-cell receptor (TCR) genes has been described in up to 30% of precursor lymphoid neoplasms, but this phenomenon occurs rarely in lymphomas with a mature phenotype. We describe three cases of immunodeficiency-associated lymphomas, two in patients with acquired immunodeficiency syndrome (AIDS) and one in a patient with a renal transplant, in which both immunoglobulin and TCR gene rearrangements were identified. Two cases exhibited clonal JH, clonal JK, and clonal TCR beta gene rearrangements; the other case had clonal JK and TCR beta gene rearrangements with deletion of JH. Two of these bigenotypic lesions exhibited a B-cell phenotype, and the other exhibited evidence of both a B- and T-cell phenotype, containing cytoplasmic CD3 as well as monoclonal kappa light chain. In addition, two of the cases exhibited the presence of clonal Epstein-Barr viral DNA. These observations suggest that lymphomas arising in immunodeficiency states may be more prone to disordered differentiation than those arising in an intact immune system.