Immunoglobulin (Ig) gene rearrangements are used to define clonality of suspected B-lineage malignancy in tissue samples. To determine whether such rearrangements could be identified in plasma, we screened plasma from 14 consecutive patients with AIDS-related lymphoma with multiplex Ig primers. Clonally rearranged Ig DNA was detected in plasma from 7 of 14 patients. Patients in whom clonal Ig DNA remained detectable after combination chemotherapy died with lymphoma. Tumor was available from 1 patient, and the IgH amplification products from plasma and tumor were sequenced and confirmed to be identical. Ig DNA rearrangements in plasma may be useful as a lymphoma-specific tumor marker, and failure to clear clonal Ig DNA may identify patients at high risk for failure of standard therapy.