We report the results of consecutive tests in nine BCR-ABL-positive ALL patients by one-step and two-step (nested primer) reverse transcriptase-polymerase chain reaction (RT-PCR). Six patients could be tested in complete haematological remission (CHR). One patient remained one-step positive; four patients became one-step negative, but remained two-step positive; only one patient became two-step negative. In five patients the haematological relapse was preceded by one-step positivity by 1.5-5 weeks. In two patients who received autologous BMT in CHR, BCR-ABL was still detectable by two-step PCR, whereas allogeneic BMT was able to transiently reduce BCR-ABL below the two-step detection level. Our results show that one-step combined with two-step RT-PCR analysis gives valuable information about the efficacy of treatment and the dynamics of the leukaemic clone.