A mechanistic marker correlating with tumor progression and clinical response is useful for assessing therapeutic response and determining the course of therapy. Since serum-total-ganglioside (sTG) and antiganglioside-IgM antibody levels reflected tumor progression, the feasibility of utilizing sTG for assessing the response to immunotherapy of metastatic-melanoma was tested. Patients (n = 34) were immunized with dendritic cells cocultured with irradiated, IFNγ-treated autologous tumor cells admixed with GM-CSF. Levels of sTG and antiganglioside-IgM antibody titers were measured in sera of vaccine recipients at 0, 4 and 24 weeks of treatment. Based on sTG-level, whether lower (L) or higher (H) than the mean + 1 SD of normal and healthy volunteers on weeks 0, 4 and 24, patients were categorized into cohorts-I (LLL, n = 16), II (HHL/HLL, n = 4), III (LLH/LHH/LHL, n = 7) and IV (HHH/HLH, n = 7). The cohorts were regrouped as sTG- downregulators (sTG-DR; n = 20) and upregulators (sTG-UR; n = 14). These two cohorts differed significantly in their overall (p < 0.012) and progression-free (p = 0.0001) survival post-treatment. 43% sTG-UR died within 39 months, with a median survival of 39 months, whereas 61% of the sTG-DR survived for 48 months. Both endogenous and vaccine-induced antiganglioside-IgM antibodies appeared to regulate sTG levels. Nonresponders had increased sTG with no or low IgM antibody response. The sTG level is regulated within 24 weeks post-treatment and therefore, may serve as an ideal biomarker for assessing therapeutic responses in patients. Clinical correlations of sTG indicate that sTG-downregulating therapy may be an effective treatment strategy for melanoma. © 2007 Wiley-Liss, Inc.