In the immunohistology of malignant melanoma the use of polyclonal antibodies against protein S100 is well established. Recently, it was shown that S100B, a subunit of the S100 protein family, is detectable in the serum of melanoma patients and correlates with the stage of the disease in patients with metastatic melanoma. In the present study, the first evaluation of a large number of treatment observations (n = 77) in 64 different patients during chemotherapy and/or immunotherapy for advanced metastatic melanoma (stage IV) is presented. All patients received treatment according to standardized protocols comprising 8 weeks of treatment followed by routine staging procedures to evaluate therapeutic outcome. In 13 patients with tumour enlargement after first-line therapy, a second-line treatment was subsequently given. S100B immunoradiometric assay (IRMA) tests were performed before, during and after treatment at scheduled time points. In the interim analysis at 4 weeks 29 of 37 (78%) patients with tumour progression during treatment showed a raised S100B level. In the final analysis at 8 weeks, 31 of these 37 patients (84%) demonstrated rising S100B values (P < 0.001). Patients who responded to treatment (stable or regressing metastatic disease) showed constant or declining S100B levels in 38 of 40 patients (95%) at the interim analysis, at 8 weeks this was further increased to 39 of 40 patients (98%; P < 0.001). Thus, the use of S100B for monitoring treatment is adequate in the majority of cases. Our observations are of great interest for therapeutic trials of adjuvant and palliative therapies as the rise of S100B levels might indicate that re-staging and/or changes in therapy strategies should be chosen.