Background: S-100 protein, commonly used in the immunohistochemical diagnosis of malignant melanoma and melanoma metastases, has recently been introduced as a tumor marker in peripheral blood. Objective: This prospective, observational, 2-center study evaluates S-100 in peripheral blood of patients with melanoma as a marker for metastasis. Methods: With application of an immunoluminometric assay, S-100 levels in 1396 samples of 570 patients with melanoma and 53 control subjects were measured in a blinded manner. Results: The cut-off level for patients with melanoma without medical history of metastases versus patients with newly occurring lymph node, visceral, and/or brain metastases was 0.114 μg/L, with a sensitivity of 94% (95% confidence interval, 86.4%-98.5%) and a specificity of 91% (95% confidence interval, 87.7%-93.6%). False-negative results included patients with unknown primary melanoma and those with amelanotic melanoma metastases. Conclusion: The data suggest that S-100 in the peripheral blood of patients with melanoma could serve as a marker indicating new melanoma metastases and could help to monitor the course of the disease. (J Am Acad Dermatol 1999;41:962-9.)