Cystathionine is not normally present in urine and only a few cases of congenital cystathioninuria are known. Cystathioninuria has also been found in patients with sympathetic tumors or with primary hepatic tumors. This study was undertaken in order to assess the significance of cystathioninuria in the diagnosis and follow-up of patients with active neuroblastoma or ganglioneuroblastoma. It was found that 50% of these patients excrete cystathionine. The presence or severity of cystathioninuria does not correlate with the extent of metastases, nor is it combined with an elevated vanilmandelic acid excretion. By contrast, patients successfully treated for neuroblastoma, as well as patients with a variety of other tumors or diseases, do not show cystathioninuria. It is concluded that cystathioninuria is a valid test in the diagnosis of neuroblastoma or ganglioneuroblastoma, although primary liver tumors should be considered in the differential diagnosis. Furthermore, the presence of cystathioninuria in the follow-up course of these patients indicates that the neoplastic disease is still active.