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Background: Fumaric acid esters (FAEs) are a broadly used therapy option for patients suffering from moderate-to-severe plaque psoriasis. Various studies in psoriasis have already shown peripheral blood lymphopenia during oral FAE therapy. Smaller studies also documented a reduction of CD4+ and CD8+ cell counts. Up to now, there are few case reports on opportunistic infections under FAE therapy - all of them associated with lymphopenia. Objective: To examine the influence of FAEs on white blood cells with special regard to leukocytes, lymphocytes, and CD4+ and CD8+ cells during psoriasis therapy. Patients and Methods: A cohort of 105 patients with diagnosed moderate-to-severe chronic plaque psoriasis was enrolled in this single-centre observational trial. For a cohort of 36 patients, T-cell subset analyses were performed. Results: Of the total, 65 patients were male (61.9%) and 40 (38.1%) female; the mean age was 43.3 years (range 16-73 years). The median lymphocyte count was significantly reduced by about 35.8% after the first 6 months of therapy. When assessing the lymphocyte count nadir over the whole period observed, 46.7% of the patients developed lymphopenia. Severe lymphopenia (Conclusion: To the best of our knowledge this is the largest clinical investigation analysing prospectively CD4+ and CD8+ cell counts in psoriasis patients receiving FAEs. We suggest periodic monitoring of absolute lymphocyte counts as well as the establishment of the determination of T-cell subsets prior to and during therapy.