Abstract: This paper reports a comparison between sister chromatid exchange (SCE) rates in lymphocytes of psoriatic patients before and after a course of PUVA treatment. The results showed that there are no significant differencas (P>0.05) in a comparison between 43 cases of psoriatic patients (SCEX±SE=6.79±0.20) before treatment, 116 healthy persons SCEX±SE=6.48±0.11) and 21 cases of non-pso-(riatic patients (SCEX±SE=6.22±0.27).It has been revealed that the level of SCE in our psoriatic patients is not so higth. In contrast to the SCE levels before PUPA treatment, the SCE levels increased remarkably (P<0.001) after PUPA treatment with an average increase of 1.91 times. It is thus considered that long-term PUPA treatment is possibly harmful for psoriatic patients.