Ten cases of angioimmunoblastic lymphadenopathy with dysproteinemia (AILD) and AILD-like lymphoma were studied by immunophenotypic and immunogenotypic analysis. All specimens were found to have a predominance of T cells by immunophenotypic analysis. DNA hybridization analyses showed three of five specimens of AILD and five of six specimens of AILD-like lymphoma to contain clonal rearrangements of the beta T-cell receptor gene. No rearrangements of the heavy or light chain immunoglobulin genes were seen in any case. A single case showed a progression of AILD with a germ-line pattern of beta T-cell receptor DNA to AILD-like lymphoma with detectable clonal rearrangements for beta T-cell receptor DNA. These results suggest that many, but not all, cases diagnosed histologically as AILD or AILD-like lymphoma contain a clonal proliferation of T-lymphocytes.