Background: The presence of an inflammatory infiltrate consisting of helper T cells and a dysregulated matrix metabolism leading to excessive deposition of collagen are two pathogenetic factors responsible for the developments of fibrosis and sclerosis in patients with systemic sclerosis. In previous studies, ultraviolet A1 (UVA1) radiation phototherapy was shown to deplete skin-infiltrating T cells through the induction of T-cell apoptosis and to up-regulate the expression of matrixmetalloproteinase-1 (collagenase-1) in dermal fibroblasts. Objective: Our purpose was to determine whether UVA1 phototherapy is effective for systemic sclerosis. Methods: Lesional skin on the forearms of patients with systemic sclerosis (diffuse type, n =3; limited type, n =1) was exposed to medium-dose UVA1 radiation (60 J/cm²) daily. Results: In all patients studied, UVA1 phototherapy-treated skin lesions were markedly softened after 9 to 29 exposures. Clinical improvement was associated with an increase in (1) joint passive range of motion values (P < .05), (2) skin temperature (thermography, P < .05), and (3) cutaneous elasticity (cutaneous elastometry, P < .05). Histologic evaluation of skin specimens obtained before and after UVA1 phototherapy revealed loosening of collagen bundles and the appearance of small collagen fibers. Conclusion: These studies indicate that UVA1 phototherapy is effective for patients with systemic sclerosis. (J Am Acad Dermatol 2000;43:670-4.)