5-S-cysteinyldopa is a precursor of melanin. Its serum and urinary level can reflect melanoma progression. In this study the concentration changes of 5-S-CD in melanomas of all stages were examined, and patients were monitored during and after treatment. Serum samples were taken from 479 melanoma patients with different Stages on 1924 occasions, from June 1996 to December 2000. Levels of 5-S-CD were determined by HPLC. The mean/median value of 5-S-CD in the Stage I-II-III patients and in the control group ranged around the normal level. Significant difference was found between Stage III and Stage IV as well as between patients with no evidence of disease and patients with tumor burden. In Stage IV 69.7% sensitivity, 61.5% specificity and 79.3% positive predictive value were evaluated. The survival of patients with normal 5-S-CD level (n= 235) differed significantly from cases having elevated marker concentration (n= 244). One hundred eighty cases were regularly monitored on 1210 occasions. Recurrence development was noticed in 57 patients. In 24.6% of these patients suffering from any type of disease progression the increasing marker level preceded the detection of metastasis by conventional methods. Serum 5-S-CD in Stage IV is sensitive enough to detect distant metastasis, and its predictive value has a great importance. It is a reliable marker for monitoring the clinical course in malignant melanoma.