The degree of tumor load reduction after therapy, which is determined by the degrees of cytoreduction and cytogenetic response, is an important prognostic factor for patients with chronic myelogenous leukemia (CML). Conventional metaphase analysis is considered to be the "gold standard" for evaluating cytogenetic response. The frequency of cytogenetic analysis can be reduced considerably if patients are monitored by molecular methods, such as quantitative Southern blot, fluorescence in situ hybridization (FISH), quantitative western blot, or competitive reverse transcriptase polymerase chain reaction (RT-PCR). Molecular methods can be performed on peripheral blood specimens and are therefore less invasive than cytogenetic analyses of bone marrow metaphases. Furthermore these techniques are applicable to Ph-negative/BCR-AbL-positive cases. Results obtained by Southern blotting, western blotting, and FISH are readily quantifiable but their sensitivity is not generally superior to that of cytogenetic methods. RT-PCR is by far the most sensitive method. Quantitative RT-PCR analysis is the method of choice for monitoring patients after bone marrow transplantation (BMT). Using competitive PCR in patients after BMT, reappearance and/or rising levels of BCR-ABL transcripts can be detected prior to relapse. All complete cytogenetic responders to interferon-alpha are positive for BCR-ABL transcripts. The level of residual disease spans a range over found orders of magnitude. In both interferon-alpha-treated patients and patients after BMT a good correlation between BCR-ABL transcript numbers per microgram of RNA and cytogenetic results has been found. Variables in the competitive PCR assay may be controlled for by quantification of transcripts of the normal ABL gene as an internal standard. We suggest a stepwise strategy for diagnosis and follow-up of CML patients employing molecular methods.