Background: UVA1 (340 to 400 nm) was found to be effective in the treatment of early-stage mycosis fungoides (MF). Objective: The purpose of this study was to assess the efficacy of UVA1 phototherapy for widespread plaque-type, nodular, and erythrodermic MF. Methods: Thirteen patients (8 with stage IB, 4 with IIB, and 1 with III MF) received 100 J/cm² UVA1 daily until remission. Four patients also had lesions inaccessible by UVA1 that were considered control lesions. Immunocytologic studies of skin infiltrates and circulating T cells were done before and after the therapy. Results: Eleven patients showed complete clinical and histologic responses. Two patients had a partial improvement. Unirradiated control lesions never improved. Serious short-term side effects were not recorded. Circulating CD4⁺/CD45RO⁺ and CD4⁺/CD95⁺ lymphocytes were significantly reduced by the therapy. Conclusion: UVA1 therapy is an effective and well-tolerated treatment for advanced MF. The therapeutic relevance of the effects on circulating lymphocytes remains to be established because lesions in nonexposed cutaneous areas did not respond.