At present there is no international consensus on laboratory testing during the follow-up of melanoma patients. We carried out a prospective monitoring study on the usefulness of lactate dehydrogenase (LDH) and protein S-100B assessment in high-risk melanoma patients. Ninety-seven patients treated within prospective randomized trials on the adjuvant treatment of melanoma received quarterly clinical visits and blood examinations. During the median observation period of 30 months disease progression was observed in 52 of 97 patients (53.1%). The clinical course of melanoma was correlated to elevated LDH and S-100B serum concentrations. The comparative analysis revealed that (i) neither LDH nor S-100B were indicators of in-transit metastases, (ii) clinically apparent lymph nodes were rarely detected because of elevated S-100B (29.4%) or LDH (11.8%) only, and (iii) the S-100B assessment was superior to LDH in the identification of early distant metastasis (53.8 vs. 23.1%; P=0.008). The rate of false-positive (elevated) LDH-serum levels and S-100B-serum levels in clinically disease-free melanoma patients did not differ significantly (S-100B 1.9% vs. LDH 1.6%). Our data indicate that only protein S-100B might be used as a highly specific and relatively sensitive marker of early distant metastasis. Both markers, LDH and S-100B, are not able to identify loco-regional metastases with a low tumor load in high-risk melanoma patients.