Narrowband UVB (NB-UVB) is a newly developed UVB source that, in addition to the previously used broadband UVB (BB-UVB), has been effectively used in phototherapy of various skin diseases. Besides its therapeutic effectiveness, NB-UVB also has some adverse effects that should be evaluated. As with all phototherapies, the photocarcinogenic potential of NB-UVB is the major concern. To assess the carcinogenic potential we measured the DNA damage induced by the two UVB sources because exposure of cells to UVB directly or indirectly induces DNA damage such as cyclobutane pyrimidine dimers (CPD) or 8-oxo-7,8-dihydro-2′-deoxyguanosine (8-oxodGuo), respectively. These types of DNA damage cause mutations of oncogenes and tumor suppressor genes, which can lead to photocarcinogenesis. In the present study we measured the yield of CPD and the oxidative DNA damage marker, 8-oxodGuo, in organ-cultured human skin and in mouse skin after exposure to NB-UVB or BB-UVB at therapeutically equivalent doses. We show that a 10-fold higher dose of NB-UVB yields a similar amount of CPD compared with BB-UVB in two types of samples examined. In contrast to CPD, the formation of 8-oxodGuo after irradiation with NB-UVB at a 10-fold higher dose is 1.5–3 times higher than that caused by BB-UVB. These results suggest that although NB-UVB at equivalent erythema–edema doses is not more potent in inducing CPD formation than is BB-UVB, NB-UVB may generate a higher yield of oxidized DNA damage.