Abstract The glucuronic acid conjugate of 3-methoxy-4-hydroxyphenylglycol(MHPG) has been isolated from human urine and its identification is described. This compound has been found to be a metabolite of dl-MHPG- 3h and of DL-norepinephrine- 14C. The quantities of MHPG, MHPG sulfate and MHPG glucuronide which are present in normal human urine, and in the urine of patients with neuroblastoma, ganglioneuroma and pheochromocytoma, indicate that the glucuronic acid conjugate is a major metabolite of MHPG. In addition, MHPG and its conjugates are markedly increased in the urine of most patients with tumours of neural crest origin. This finding provides further evidence that MHPG is the major metabolite formed within these tumours, and the assay of urinary MHPG, especially after enzymatic hydrolysis of the glucuronide and sulfate conjugates, should be of value in the diagnosis of tumors of neural crest origin.