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 7 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-β (TCR-β) 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. © 1992 Raven Press, Ltd., New York.