The majority of our genes may be regulated in a daily rhythm, including the genes for cell cycle control. Epidemiological and genetic evidences suggest that disruption of circadian timing mechanisms makes our cells more vulnerable to cancer formation. The aim of this study was to investigate the relationship between expression patterns of circadian clock genes (period homolog (per)1, per2, clock, and cry1) and tumor development by analyzing human skin biopsies of malignant melanoma and nonmalignant naevus tumors. We found that mRNA levels and nuclear immunopositivity for the investigated clock genes were reduced by 30-60 % in both melanoma and in naevus biopsies if compared with adjacent nontumorous samples. The alterations in melanoma presented significant associations with clinicopathological characteristics (e.g., Breslow thickness). Contrary to previous reports, the moderate decrease of per1 expression seen in malignant tissues could not be linked to malignant transformation itself; rather, it reflects only the alterations in tissue composition. In turn, clock expression was upregulated in nontumorous cells of melanoma biopsies but not in melanoma cells or naevus cells. As this gene (clock) is closely related to cellular metabolism, our data suggest its role in the impaired regulation of metabolism in malignant tumors. Our results present the first clinical evidence for a possible link between circadian clock genes and human skin tumorigenesis.