MYCN oncogene amplification is an established indicator of the aggressiveness of neuroblastomas; it is used internationally for stratifying patients for therapy. The present study shows that high levels of MYCN DNA sequences are present in the peripheral blood of patients with MYCN-amplified neuroblastomas. Circulating MYCN DNA may be a powerful and noninvasive prognostic marker at the time of diagnosis. Furthermore, preliminary data strongly suggest that the release of MYCN sequences in the peripheral blood is an early process in disease progression, permitting us to propose this novel marker for the follow-up of patients after chemotherapy.