Oxidative stress may play an important role in the pathogenesis of psoriasis. Glutathione S-transferases (GSTs) make up a group of antioxidant enzymes. Cytochrome p450 (CYP) enzymes can influence oxidation and reduction reactions. We investigated the potential effects of GST and CYP enzymes in the pathogenesis of psoriasis. The study included 32 psoriasis patients and 22 healthy subjects. Psoriasis patients were administered 20 sessions of narrowband ultraviolet B phototherapy. Expressions of GST and CYP enzymes were assessed by immunohistochemical staining. Expression levels of GSTK1, GSTM1, and GSTT1 were significantly higher in psoriasis than in control tissues (P = 0.022, P = 0.001, and P = 0.006, respectively). Pre- and post-treatment expression was similar. Expression of CYP1A1 and CYP2E1 was significantly higher in pre- (P = 0.003 and P = 0.001, respectively) and post-treatment (P = 0.003 and P = 0.001, respectively) psoriatic tissues than in control tissues. No significant differences in CYP1B1 levels between the study and control groups were detected before treatment (P > 0.05). However, CYP1B1 levels were higher in post-treatment psoriatic tissue than in control tissue (P = 0.045). The significant increases in expression of GSTK1, GSTM1, and GSTT1 in psoriasis may reflect the increased activation of GST in response to excessive free radical formation from activated neutrophils or ultraviolet exposure to maintain antioxidant capacity in psoriasis. Furthermore, expressions of CYP1A1 and CYP2E1 represent important enzymatic systems in psoriasis. These findings suggest that psoriasis is an oxidative stress condition, although phototherapy does not affect these enzymatic systems. Further investigation is required.