The relative induction of cyclobutane pyrimidine dimers (CPD) and pyrimidine (6–4)pyrimidone photoproducts ([6–4]PD) was quantified in the duplex homopolymers polydeoxyadenosine : polydeoxythymidine, polydeoxyguanosine : polydeoxycytidine and polydeoxyguanosine :polydeoxy-5-methylcytidine irradiated with UVC or UVB radiation. Cytosine methylation significantly increased the yield of cytosine (6–4)PD after irradiation with UVC light and of cytosine CPD and (6–4)PD after irradiation with UVB light. The data suggest that CPD and (6–4)PD are preferentially induced at 5-methylcytosine bases in DNA of cells exposed to sunlight and comprise a major component of the mutation spectrum leading to the initiation of sunlight-induced skin cancer.