To detect occult lymphoma, the polymerase chain reaction (PCR) technique was used to amplify joined bcl-2/JH DNA sequences at the juncture of the t(14:18) translocation in follicular lymphoma. Using the heat-stable DNA polymerase Taq and automated cycling of the reaction, we were able to detect as few as one to two copies of bcl-2/JH. Under these conditions, PCR proved to be at least 10,000-fold more sensitive than either conventional flow cytometry or Southern blot restriction analysis. In addition, genomic DNA sequences of four lymphomas confirmed that the size of the amplified segment serves as a tumor marker. Direct application of PCR to patient staging revealed occult malignant lymphoma in tissue otherwise considered uninvolved by standard criteria. We conclude that the striking enhancement in diagnostic sensitivity attained by DNA amplification can serve as a valuable adjunct to the staging and clinical monitoring of follicular lymphoma.