PRAME (Preferentially expressed antigen of melanoma) has been previously identified as a melanoma antigen recognized by cytotoxic T cells (CTLs) and found to be expressed in a variety of cancer cells including leukaemic cells. We have screened 98 Japanese patients with leukaemia and lymphoma for expression of the PRAME gene using semiquantitative reverse transcription polymerase chain reaction (RT-PCR). Forty-one patients (42%) showed high levels of PRAME expression. Eight of these patients were then monitored using real-time PCR for a period of 10–37 months. Significant reductions in the PRAME expression were observed in all patients after chemotherapy. An increased expression was detected in the two patients who relapsed, one of which was before cytological diagnosis. These changes were correlated with those of other known genetic markers, such as the bcr-abl gene. Therefore, quantitative monitoring of the PRAME gene using real-time PCR method may be useful for detecting minimal residual disease and to predict subsequent relapse, especially in patients without known genetic markers. In addition, a PRAME-positive leukaemia cell line and fresh leukaemic cells were found to be susceptible to lysis by PRAME-specific CTLs established from a patient with melanoma, suggesting that the PRAME peptide can also be a target leukaemia antigen for T cells.