A series of T-cell proliferations in peripheral blood, bone marrow, or tissue samples, together with seven T-cell lines, were analysed for clonality. The technique used employs the polymerase chain reaction (PCR) to amplify rearranged T-cell receptor gamma genes, using primers recognising conserved sequences in the variable and joining gene segments. Of the 20 cases of T-cell leukaemia or lymphoma analysed, a clone was detected in 14 (70%): Of seven T-cell lines, a clone was detected in 6 (84%). No positive results were recorded in eight non-T-cell disorders (including nonlymphoid malignancies and reactive disorders). When the results of this technique were combined with the results of our previously published method for the detection of clonally rearranged T-cell receptor-beta (TCR-beta) genes using PCR, 9 of 10 (90%) T-cell tumours were detected. This method uses only four primer combinations in two tubes, and is therefore simple and rapid: it requires no radiolabelling, uses only a small amount of tissue, and can be performed on formalin-fixed, paraffin-embedded tissue.