Allelic deletion of the long arm of chromosome 11 (11q loss) is closely associated with the prognosis of neuroblastoma (NB). Here we examined 11q loss using tumor-released DNA fragments in the sera of 24 cases. The allelic intensity score of a panel of polymorphic markers in 11q23 in serum DNA was significantly different between the 11q loss-positive group and the11q loss-negative group. The 11q loss-positive and -negative groups did not overlap when a cut-off value of 0.5 was chosen for the allelic intensity score. Our serum-based 11q loss analysis could predict the allelic status of 11q in tumors.