The distinction between benign polyclonal and malignant monoclonal lymphoid disorders by morphology or immunophenotyping is frequently difficult. Therefore, the demonstration of clonal B-cell or T-cell populations by detecting identically rearranged immunoglobulin (Ig) or T-cell receptor (TCR) genes is often used to solve this diagnostic problem. Whereas the detection of rearranged Ig genes is well established, TCR gamma (γ) and beta (β) gene rearrangements often escape detection with the currently available polymerase chain reaction (PCR) assays. To establish a sensitive, specific, and rapid method for the detection of rearranged TCR-β genes, we developed a new PCR approach with family-specific Jβ primers and analyzed the resulting PCR products by high-resolution GeneScan technique. The superior efficiency of this new method was demonstrated by investigating 132 DNA samples extracted from lymph node and skin biopsy specimens (mostly formalin fixed) and blood samples of 62 patients who had a variety of T-cell lymphomas and leukemias. In all but 1 of the tumor samples (98.4%) a clonal amplificate was detectable after TCR-β PCR and the same clonal T-cell population was also found in 15 of 18 (83%) of the regional lymph nodes and in 7 of 11 (64%) of the peripheral blood samples. Direct comparison of these results with those obtained currently by the most widely applied TCR-γ PCR revealed an approximate 20% lower detection rate in the same set of samples than with the TCR-β PCR method. These results indicate that the new TCR-β PCR is most suitable for a rapid and reliable detection of clonal T-cell populations.