A patient presenting with marked inflammatory lymphadenitis and Jaccoud's arthritis was found to have a rearranged gene for the beta-chain of the T-cell receptor (TCR-beta) antigen in the lymph node DNA digests and normal germ line DNA in the peripheral blood lymphocytes. Four months later, the patient was diagnosed to have poorly differentiated adenocarcinoma of the lung with small foci of metastatic tumor in lymph nodes that contained the same extensive lymphocytic and inflammatory cell infiltrates noted earlier. Rearranged TCR-beta chain genes were detected in both lymph node and peripheral blood lymphocyte DNA at this time. The most likely explanation for the florid lymph node reaction and the unusual arthropathy appears to be a paraneoplastic immune response. The rearranged TCR-beta genes indicate a clonal T-cell expansion that most likely resulted from the aberrant immunologic response to the lung cancer.