Neuroblastoma (NB) is the most common malignant solid tumor in childhood and, among all childhood malignancies, is second only to leukemia. NB originates before birth in the neural crest, which develops into the adrenal medullae and sympathetic ganglia. In the adrenal medulla, tyrosine hydroxylase (TH) is the first enzyme in the pathway of catecholamine synthesis. We used reverse transcription polymerase chain reaction (RT-PCR) to examine the expression of TH mRNA in NB and Ewing's sarcoma cell lines, small round cell tumors (SRCTs) containing NB, and other clinical tumor samples (osteosarcoma, osteochondroma, and Wilms' tumor). In total, we analyzed 33 clinical tumor samples. TH mRNA was expressed in all three NB cell lines examined, but not in two ES cell lines or in a breast cancer cell line. We detected TH mRNA in 23 of 25 NB tumor samples (92%), but in none of the SRCTs or other clinical tumor samples. This RT-PCR technique showed a sensitivity for TH mRNA of one NB cell per 10(5) negative cells. Based on these results, the detection of TH mRNA is very useful both as a tumor marker for NB and for detecting minimal residual disease. Therefore, we can use this method to detect tumor cell contamination before hematopoietic stem cell transplantation.