Neuroblastoma is a solid tumor occurring usually in children less than 5 years old. It has been difficult to distinguish neuroblastoma from other childhood tumors through morphological diagnosis. Urine homovanillic acid (HVA), which is a metabolite of dopamine, has been proposed as a diagnostic index. Although increased levels of a serotonin metabolite, 5-hydroxyindole-3-acetic acid (HIAA), have also been observed in urine samples of the patients, they were largely attributed to dietary amines. By using an HPLC system with electrochemical detection, which can simultaneously assay 12 monoamines and metabolites, we showed that HVA and HIAA are two of the most prominent monoamine metabolites in the medium after a neuroblastoma cell line (IMR-32) was cultured for 3 days. Moreover, we found that the levels of HVA and HIAA in the media are proportional to the cell densities. These results suggest that the levels of HVA and HIAA in tissue culture media, or in urine from patients whose dietary amines are well controlled, may provide a valuable diagnostic index for neuroblastoma.