Various topical agents used in combination with phototherapy have blocking effects, however in contrast to in vitro studies there were not enough in vivo studies about this subject. Our purpose was to examine the photoprotective effects of white petrolatum and salicylic acid which can be used before UVB therapy in psoriasis patients. In 35 volunteers, a phototest was performed to determine the minimal erythema dose (MED) and the test was repeated with thin (0.1 cc/25 cm2) and thick (0.3 cc/25 cm2) petrolatum, thin and thick salicylic acid (20%) in petrolatum and sunscreen. After 24 hrs, the effects of each agent on MED was investigated. MED values detected after pure UVB and after application of topical agents were compared one by one, and the differences between all of them have been found statistically significant. These showed that MED values were increased by thin or thick petrolatum and also by thin and thick salicylic acid (20%) in petrolatum. The highest MED values were detected with thick salicylic acid in petrolatum followed by thin salicylic acid in petrolatum, thick petrolatum and thin petrolatum. The application of petrolatum and salicylic acid, which can block UVB, before phototherapy is not recommended.