Molecular detection of microcirculating or microdisseminated disease (MDD) with a sensitive methodology could contribute to a better treatment for children with neuroblastoma. To detect circulating neuroblastoma cells, we developed a quantitative assay for the analysis of tyrosine hydroxylase (TH) gene expression. We analyzed 155 samples of peripheral blood (PB) from 25 patients with neuroblastoma in advanced stages (8 stage III and 17 stage IV). TH mRNA was analyzed by RT-PCR assay using TaqMan technology. PB samples (n=25) from donors were used for normalizing TH, and values <7 were considered negative. With a median follow-up of 40 months (range 15-73 months), 9 patients relapsed and 8 patients died of progressive disease. TH expression was detected in the PB of 16 patients (64%) at diagnosis. During treatment, 10 patients had positive samples and 9 patients were still positive for circulating tumor cells at the end of treatment. Actuarial 3-year event-free survival of patients with PB positive for TH mRNA after induction therapy (40%) (p=0.018) and at the completion of treatment (33%) (p=0.003) were significantly worse than the survival of TH-negative patients (86 and 87%, respectively). In multivariate analysis, MYCN status and TH expression in PB at the end of treatment remained significant prognostic factors. Our results show that patients with advanced neuroblastoma who have PB positive for TH at the end of treatment seem to have a worse prognosis compared with patients with undetectable TH. These results suggest the usefulness of MDD monitoring in neuroblastoma.