Xeroderma pigmentosum (XP) patients are clinically characterized by a very high incidence of skin cancers on exposed skin, at an early age. XP cells in vitro are strongly deficient in excision-repair and highly mutagenized by UV light. We were, therefore, interested in measuring mutation frequency and in determining mutation spectra in patients' tumors exposed to UV lesions. We chose to look at oncogene activation in skin tumors with the idea that more mutations, particularly of the ras gene family, would be found in XP tumors where lesions remain unrepaired compared to normal individuals. Our results clearly show that more than a 2-fold significantly higher mutation frequency (50%) of the ras genes was found in XP in contrast to control tumors (22%). The majority of the mutations were found at codon 12 of all three ras genes with a preponderance for N-ras in XP samples. The mutation spectra indicate that all mutations found were located opposite pyrimidine-pyrimidine sequences which represent a hot spot for UV-induced DNA lesions. Most of the mutations were of the type expected from studies performed in vitro with model systems. This high mutation frequency in XP was accompanied by a very high level of Ha-ras and c-myc gene amplification and rearrangement. All these data are consistent with a fundamental role of unrepaired UV-induced DNA lesions as an initiating event in human skin tumors on exposed parts of the body.