KiSS1 is a putative melanoma metastasis suppressor gene, the expression of which may be regulated by another gene(s) mapping to chromosome 6q16.3-q23. To additionally elucidate the role of KiSS1 in the progression of human melanoma in vivo, we examined KiSS1 mRNA expression in 51 melanocytic tumors with various stages of progression by in situ hybridization. We also examined a correlation between loss of KiSS1 mRNA expression and loss of heterozygosity (LOH) of 6q16.3-q23 in 27 melanoma metastases. All of the four nevocellular nevi and eight primary melanomas <4 mm in thickness showed KiSS1 mRNA expression, whereas only 50% (6 of 12) of primary melanomas >4 mm in thickness expressed KiSS1. Loss of KiSS1 mRNA was equally frequent in metastases; 44% (12 of 27) of tumors lost KiSS1 expression. LOH of 6q16.3-q23 was observed in 52% (14 of 27) of metastases. There was a strong association between LOH and loss of KiSS1 expression (P = 0.03); nine metastases with LOH of 6q16.3-q23 lost KiSS1 expression, whereas 10 tumors with no LOH showed positive KiSS1 mRNA expression. The findings in this study show, for the first time, KiSS1 down-regulation during the progression of melanoma in vivo and strongly suggest that inactivation of a tumor suppressor gene(s) mapping to 6q16.3-q23 by deletion or mutation coupled with LOH may lead to the down-regulation of KiSS1.