Serial assays of qualitative (multiplex and nested) and quantitative PCR were carried out for detecting and estimating the level of BCR-ABL transcripts in 39 CML patients following bone marrow transplantation. Seven of these patients, who received donor lymphocyte infusions (DLIs) following to relapse, were also monitored. Quantitative estimates of BCR-ABL transcripts were obtained by co-amplification with a competitor sequence. Estimates of ABL transcripts were used, an internal control and the ratio BCR-ABL/ABL was thus estimated for evaluating the kinetics of residual clones. Twenty four patients were followed shortly after BMT; two of these patients were in cytogenetic relapse coexisting with very high BCR-ABL levels while other 22 were in clinical, haematologic and cytogenetic remission 2-42 months after BMT. In this latter group, seven patients showed a favourable clinical-haematological progression in association with molecular remission while in 14 patients quantitative PCR assays indicated molecular relapse that was not associated with an early cytogenetic-haematologic relapse. BCR-ABL/ABL levels could not be correlated with presence of GVHD in 24 patients after BMT. In all seven patients treated with DLI, high levels of transcripts were detected at least 4 months before the appearance of clinical haematological relapse. Following DLI, five of these patients showed decreasing transcript levels from 2 to 5 logs between 4 and 12 months. In eight other patients studied long after BMT, five showed molecular relapse up to 117 months post-BMT and only one showed cytogenetic relapse. Our findings indicated that quantitative estimates of BCR-ABL transcripts were valuable for monitoring minimal residual disease in each patient.