Techniques currently available enable the detection of clonal rearrangements of the immunoglobulin heavy chain gene using fluorescent PCR technology. It is possible to use this technique to analyse minimal residual disease throughout patient treatment: however, without the development of a quantitative assay, only the presence or absence of a clonal population can be determined. We describe here the development of a quantitative competitive PCR technique using genomic DNA which enables the rate of clearance of disease to be measured. In future, the ability to detect and also quantitate minimal residual disease may enhance the application of molecular investigations in the clinical management of patients.