Signal transducer and activator of transcription 3 (STAT3) protein has been documented as a significant mediator of interferon (IFN) signaling. Physiological STAT3 phosphorylation involves tyrosine (Y705) and serine (S727) activation. Impairment of STAT3 protein levels and/or of STAT3 phosphorylation after IFN treatment has been found in many pathological conditions such as cancer, immunopathy and inflammatory disease. To analyze tumor-associated defective STAT3 response to IFNs, the induction of S727 and Y705 STAT3 activation after IFN exposure was evaluated in 18 human malignant melanoma cell lines and 68 primary cell cultures established from the lymph node metastases of melanoma patients. STAT3 expression and STAT3 phosphorylated forms were assayed by Western blot analysis employing specific STAT3 antibodies. All melanoma cell lines as well as samples derived from metastatic melanoma patients expressed STAT3 with variable signal intensities depending on the appropriate cell type. Significantly altered IFNγ-induced S727 STAT3 activation was found in both experimental models, with on average 94.1% of patients detected to be non-responders in lymph node cell cultures and 83.3% in melanoma cell lines. Moreover, a deficiency in IFNα-induced S727 induction was detected in 88.9% of melanoma cell lines. Defects in Y705 STAT3 phosphorylation were determined in clinical material (61.8% after IFNγ exposure) as well as in melanoma cell lines (absence of response to IFNα/γ in 83.3 and 55.5%, respectively). Our data clearly confirm STAT3 pathophysiological perturbances in human malignant melanoma cells. Depending on the induction of STAT3-activated phosphoforms by IFNs, three categories of melanoma cells were identified: a) phosphorylation on both the S727 and Y705 amino acid residues; b) STAT3 activation on Y705 only; c) phosphorylation at neither S727 nor Y705. The significance of in vitro STAT3 activation for predicting patient response to immunotherapy will be examined in a prospective clinical study by our group.