Melanoma is a highly prevalent cancer that is associated with substantial mortality. Although clinical staging procedures can serve as relatively robust prognostic indicators, we aimed to determine whether assessments of the abundance of hypoxia inducible factor-1α (HIF-1α) or vascular endothelial growth factor (VEGF) in postexcisional melanoma tumor tissues may enable more accurate determination of tumor aggressiveness. We carried out a multicenter prospective study, in which we systematically evaluated 376 consecutive patients diagnosed with melanoma, and performed histochemical assessments for both HIF-1α and VEGF immunoreactivity in the tumor biopsies. Multivariate analyses showed that higher HIF-1α expression, but not high VEGF, were associated significantly and independently with increased tumor aggressiveness as derived from several well-established aggressiveness criteria. A limitation of this study was that this was a descriptive prospective study lacking a post-hoc verification arm. Thus, the presence of increased numbers of positively labeled HIF-1α cells in melanoma tumors may potentially serve as an indicator of tumor phenotype and prognosis, and accordingly guide therapy.