A competitive polymerase chain reaction (PCR) technique was developed to quantify residual malignant cells in the peripheral blood and bone marrow of patients with low-grade follicular non-Hodgkin's lymphoma carrying a translocation between chromosomes 14 and 18. Artificial segments were constructed imitating a translocation between chromosome 14 and 18. These artificial translocation segments were used as competitor molecules in the quantitative PCR. Serial dilutions of a known amount of patient-derived translocation segments were coamplified with a fixed number of competitor molecules, and a patient specific calibration curve was constructed. Several patient samples were coamplified with an equal number of competitor molecules and the number of t(14;18) translocations within the samples was calculated by comparison with the calibration curve. The method was demonstrated on samples of four follicular non-Hodgkin's lymphoma (NHL) patients. In a patient transplanted with allogeneic bone marrow declining numbers of residual lymphoma cells were observed. We conclude that the method is accurate, relatively fast and the general principle of this method can be applied to all malignancies with characteristic abnormalities on DNA or RNA level that are detectable by PCR.