We have studied the effect of light intensity on the production of (pre)cancerous lesions in SK-1 hairless mice which were irradiated with broad band solar-simulating radiation (290-400 nm). Four groups of 16-20 animals were chronically irradiated 5 times per week, starting with 6.3 J/cm² (0.9 minimal erythemal dose) followed by incremental increases in the radiation dose by 20% of the original dose every tenth irradiation day for 8 weeks (total dose 334 J/cm²) Intensities were varied among the groups by use of calibrated Oriel neutral density filters, which afforded a 12-fold variation of emitted radiation intensity. Clinical and histological results indicated that the development of (pre)cancerous lesions (squamous cell carcinoma) was favored, although not markedly so, by higher intensities. The present result indicates that the photochemical (“light”) reactions which lead to carcinogenesis are much faster than the opposing “dark” reactions which lead to regression. These results are different from those of previous studies, which showed that carcinogenesis is markedly favored by more aggressive dose fractionation at constant intensity.