Fifty patients were included in this study (20 with mycosis fungoides, 20 with psoriasis and 10 controls). Skin biopsies were taken from the cutaneous lesions and one from each control. They were used to measure the levels of Fas and Fas ligand by polymerase chain reaction. The results showed loss of Fas expression in plaque and tumor stage mycosis fungoides indicating tumor progression where Fas was only detected in the patch stage. As for Fas L, it was detected in 90% of cases of mycosis fungoides with no relation to the stage of the disease. Fas ligand followed Fas, where levels of Fas ligand were higher in patch than plaque and tumor. As for psoriasis, Fas was detected in all biopsies indicating a possible role for Fas in the development of psoriasis away from the apoptotic pathway. Fas ligand was only detected in 40% of cases of psoriasis, explaining the defective apoptosis where the low level of Fas ligand leads to improper Fas activation. Thus, Fas/Fas ligand system can be used as a predictor for the prognosis of mycosis fungoides and can add to the pathogenesis of psoriasis