The objective of this paper is the modelization of the influence that the treatment with optical brighteners has on the ultraviolet protection factor (UPF) of fabrics whose composition and structure are appropriate for use in summer garments. Following an experimental design, the study has been conducted with fabrics composed of three different cellulosic fibers (cotton, Modal, and Modal Sun) and with three different structures. The fabrics have been treated with two optical brighteners at several concentrations. A statistical model has been formulated for each fiber type, which allows estimation of the UPF, with regard to the type of optical brightener and its concentration and the initial UPF of the untreated fabric.