The molecular events responsible for tumor progression in human cutaneous malignant melanoma remain unclear; however, critical to the process is the dysregulated proliferation of tumor cells and the development of new vascular channels which allow further growth and dissemination. Connective tissue mast cells (MC) have been implicated in tumor progression because they concentrate around tumors (including melanomas) prior to the formation of new blood vessels, and because they contain many chemical mediators, including basic fibroblast growth factor (bFGF), known to have mitogenic and angiogenic effects. Several MC chemotactic and mitogenic factors have been described including interleukin-3 (IL-3). In order to determine whether there is a differential expression of this MC chemotactic/mitogenic factor with tumor progression in vivo, we evaluated by immunohistochemistry 85 melanocytic lesions including primary invasive malignant melanoma (PIMM), melanoma in situ (MMIS), and ordinary intradermal benign melanocytic nevi (BMN) for expression of IL-3. Nucleic acid in situ hybridization also was used to evaluate the melanocytic lesions for IL-3-specific mRNA transcripts. Intracellular IL-3 protein was detected in 29/33 (88%) PIMM and 15/25 (60%) MMIS, but was not detected in any (0/27; 0%) BMN (p<0.0001). IL-3-specific mRNA transcripts were present in 3/4 PIMM and 2/10 MMIS in which IL-3 protein was not identified, but were not detected in any BMN. IL-3 mRNA or protein was not detected in normal melanocytes present in the perilesional epidermis of any of the specimens studied. Immunohistochemistry also was used to confirm the presence of IL-3α-specific receptors on human cutaneous MC. As demonstrated by others, a significantly increased number of MC was present in the perilesional stroma of PIMM and MMIS vis à vis BMN (p<0.0001). The results suggest that melanoma cells may attract MC in vivo by producing MC chemotactic/mitogenic factors such as IL-3. The recruitment of MC and the subsequent release of their potent mitogenic and angiogenic factors such as bFGF may thus represent a tumor–host interaction which favors tumor progression.