Granulomatous slack skin (GSS) is an extremely rare disorder within the group of cutaneous T cell lymphomas (CTCL). Ultraviolet A1 (UVA1) phototherapy has previously been reported to be useful in the treatment of CTCL such as mycosis fungoides. We report a 35-year-old Caucasian male with GSS treated with UVA1 phototherapy starting at 20 J/cm(2) UVA1 3 times a week and subsequently increased in increments of 5 J/cm(2) to a medium-range dose of 50 J/cm(2) per session. The patient underwent a total of 45 sessions with a cumulative dose of 1,495 J/cm(2) UVA1 without any adverse events. At the conclusion of UVA1 phototherapy, a decrease in erythema and skin thickness was observed which was most prominent in the periphery of the lesion in the right groin area. A follow-up 12 months after phototherapy showed continued treatment benefit. To our knowledge, this is the first report describing the successful use of UVA1 (340-400 nm) phototherapy in a patient with GSS.