Recently, Ewing's tumours have been shown to carry specific hybrid transcripts resulting from the fusion of the EWS gene with FLI-1 or ERG genes. Based on the sensitivity and specificity of the detection of these alterations by the reverse transcriptase-polymerase chain reaction technique, we have developed an assay to search for small numbers of Ewing cells in various sites from patients with Ewing's tumour. This method enables the detection of fewer than one tumour cell per million blood mononuclear cells. A total of 28 primary sites and 51 peripheral samples from 36 patients were investigated. Tumour cells could be detected in 4/18 blood samples, 4/15 bone marrow aspirates and 2/18 peripheral stem cell harvests. EWS/FLI-1 and EWS/ERG transcripts being observed in eight and two cases respectively. The type of fusion transcript detected in peripheral site(s) was identical to that observed in the primary site. At diagnosis 5/16 patients (31%) demonstrated either circulating tumour cells or/and occult bone marrow metastasis. After induction therapy, tumour cells were detected in 3/21 patients. This highly sensitive method should be a relevant tool to allow a more accurate clinical assessment of the dissemination of Ewing's tumours.