Cytogenetic and pathologic studies were performed on six patients with angioimmunoblastic lymphadenopathy (AILD). All six had diffuse lymphadenopathy; five had fever, four had weight loss, and four had a diffuse erythematous rash. All patients except one had a polyclonal elevation of immunoglobulin. All patients had diagnostic findings in lymph node (LN) and bone marrow (BM) biopsies. Two patients died of progressive AILD; one patient died after transformation of AILC to immunoblastic sarcoma (IBS); one patient died of gastrointestinal bleeding of unknown cause. The remaining two patients, who have achieved complete remission with intensive chemotherapy, are alive 20 and 8 mo after the diagnosis; one of these had AILD and the other, both AILD and IBS. Despite diagnostic BM biopsy findings, none of the patients had chromosome abnormalities in their BM cells. In studying LN cels of 5 patients, however, we found chromosome abnormalities in each; clonal abnormalities were detected in two, both clonal and nonclonal abnormalities in two, and only nonclonal single-cell abnormalities in one. An extra chromosome 3, seen in four patients, was clonal in two and nonclonal in the two others. Cells with +5, +15, +19, +21, +22 were seen in two patients. All patients had 50% or more normal dividing cells in their LN. The mosaicism of unrelated abnormal cells in their LN. The mosaicism of unrelated abnormal karyotypes that was seen in four patients suggests that this malignant tumor is not necessarily monoclonal in its early stages, but that one clone may be selected and predominate in the late stage. Because nonrandom acquired clonal chromosome abnormalities are a consistent feature of malignancies, our data suggest that AILD may be a malignant disease despite its original description as a benign proliferative process. Therefore, it may require aggressive chemotherapy.