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Background: Early and precise diagnosis of cutaneous T-cell lymphomas (CTCL) is challenging. Currently, polymerase chain reaction (PCR)-based clonality assessment of the T-cell receptor (TCR) is a helpful tool for this diagnosis. Objectives: In this retrospective study, we aimed to assess the sensitivity and specificity of this method for the diagnosis of CTCL. Materials and Methods: Monoclonal rearrangement of the TCR was investigated retrospectively by PCR-based clonality assessment based on 292 DNA samples from skin biopsies of patients with a suspicion of CTCL. Algorithms were based on different ratios (three or five-fold difference) between the dominant PCR peak and the third highest PCR peak. Results: A PCR peak five-fold higher than the third highest PCR peak demonstrated significantly greater specificity (83.7% versus 76.4%) but lower sensitivity (59.8% versus 68.6%) compared to a cut-off of three-fold higher. Conclusion: Our results confirm the diagnostic value of TCR clonality analysis which may be used to define the ideal cut-off in order to optimize sensitivity and specificity.