Objective: The diameter of purified protein derivative skin test (PPD) induration in evaluation of positive reaction varies according to patient's immune status, living area and BCG vaccination. A needle prick trauma and PPD extract may lead to Koebner phenomenon after 72 hours and these applications may be the cause of false positive or increase of diameter of PPD reaction in psoriatic patients. The aim of the study was to evaluate the results of PPD skin test in psoriatic patients and to compare with control subjects. Material and Methods: The test was performed on 117 consecutive patients with psoriasis vulgaris and 102 immunocompetant control patients. The Mantoux method was used for the test and skin reaction was measured at 72 hours. A positive skin reaction was considered > 10 mm. Results: A positive PPD test was detected in 49 psoriatic patients (41.8%) and in 31 of the control subjects (30.3%). Differences in positive PPD reactions (p=0.16) and the diameter of PPD indurations (p=0.36) between two groups were not statistically significant. The diameter of PPD indurations in psoriatic patients was not associated with sex, duration of the disease or psoriasis area and severity index score (p> 0.05). Conclusion: Our results show that psoriasis has no effect on the PPD skin test. We think that this test is appropriate to recognize latent tuberculosis infection in patients with psoriasis until more sensitive diagnostic tests become available.