Production of cytokines (CKs) in the tumor micro-environment may modulate tumor-host interactions. However, pre-clinical models often provide conflicting data and there is no established role for CKs as modulators of the natural or treatment-related behavior of tumors. Serial sampling by fine-needle aspirates (FNAs) of identical metastases from patients affected with metastatic melanoma and undergoing IL-2-based vaccination allowed prospective measurement of IL-10, TGF-beta1, TGF-beta2 and IFN-gamma transcriptional levels assessed by quantitative real-time PCR. Thus, it was possible to prospectively document the expression of markers relevant to a given treatment and follow at the same time the clinical outcome of the lesions left in place. Eight of 27 metastatic lesions completely regressed in response to the treatment and 1 demonstrated >50% shrinkage. These regressions occurred after the follow-up FNA had been obtained. IL-10 transcript was differentially expressed in pre-treatment FNA of responding lesions (t-test p(2) = 0.002). During treatment, INF-gamma transcript levels significantly increased in regressing compared to non-regressing lesions (t-test p(2) = 0.03). These data suggest that the pre-treatment CK profile of the tumor micro-environment may determine clinical responsiveness to immune therapy. Furthermore, temporal changes in CK expression during treatment might describe the biological characteristics of an effective immune response.