Quantification of occult circulating tumour cells in blood or bone marrow (BM) enables the identification of patients with a high risk for relapse in nucleophosmin/anaplastic lymphoma kinase (NPM-ALK)-positive anaplastic large cell lymphoma (ALCL). We have developed a flow cytometric (FCM) assay to quantify the rare ALK- and CD30-positive ALCL cells. When ALCL cells were admixed with normal peripheral blood or BM, ALK- and CD30-positive cells could be detected above background level at an added concentration of 10(-5) for all three cell lines tested. Sensitivity and costs of the assay were compared with quantitative real-time polymerase chain reaction (PCR) for NPM-ALK. The results of the FCM assay and quantitative PCR for NPM-ALK correlated. The sensitivity of the PCR exceeded that of the FCM by at least one log. Quantitative PCR was more time-consuming and expensive than FCM. Both methods were compared on BM or blood samples from 11 ALCL patients. FCM using antibodies against ALK and CD30 can sensitively and specifically detect the circulating ALCL cells in BM or blood. This method needs to be tested in a larger cohort of patients to determine whether it has sufficient sensitivity to be used as a substitute for quantitative PCR.