Recently, numerous studies have been reported concerning the treatment of early-stage mycosis fungoides (MF) with narrowband (311-nm) UVB, claiming a beneficial response. We have used for the first time a 308-nm monochromatic excimer light, a new kind of xenon-chloride lamp, in the treatment of patch stage IA MF. We treated 7 patch lesions in 4 patients with unequivocal clinicopathologic diagnosis of MF. All lesions achieved clinical and histologic complete remission. The number of weekly sessions varied from 4 to 11 (mean 6.5; median 5.5). The total UVB 308-nm irradiation dose ranged from 5 to 9.3 J/cm² (mean 7.1 J/cm²; median 7 J/cm²). All lesions remained in stable complete remission after a follow-up of 3 to 28 months. No remarkable side effects were reported. Our preliminary results suggest that monochromatic excimer light phototherapy is a possibly very useful treatment modality in patch stage IA MF.