Background/Purpose: Atopic dermatitis (AD) is characterized immunohistochemically by a high number of skin infiltrating T-helper cells (CD4+). In most cases cutaneous T-cell lymphoma (CTCL) is characterized by a malignant proliferation of CD4+ T-helper lymphocytes. The purpose of our study was to evaluate the extent of anti-apoptotic effects in patients suffering from AD or CTCL, respectively, which may contribute to the prolonged inflammation. Furthermore, we investigated whether medium-dose ultraviolet A1 (UVA1) phototherapy is able to modulate the expression of bcl-2 within the dermal inflammatory infiltrate. Methods: In order to enumerate bcl-2+ cells pre- and post-therapeutic punch skin biopsies of ten patients with AD and five patients with CTCL were stained immunohistochemically for features of apoptosis using a monoclonal antibody detecting bcl-2. Results: Both AD and CTCL sections revealed a high percentage of bcl-2+ cells within the dermal perivascular infiltrate before therapy. After the successful treatment using medium-dose UVA1 phototherapy this percentage could be decreased significantly. Conclusion: Both T-cell-derived skin diseases exhibit an increased pre-therapeutic number of bcl-2+ cells. After medium-dose UVA1 phototherapy the substantial improvement of the skin condition was linked to a significant decrease of the dermal bcl-2+ cell count. Moreover, we could demonstrate a remarkable correlation referring to the decrease and staining pattern of bcl-2 between these two groups as well as within each group. Because the bcl-2 protein is known to act as an apoptosis inhibitor, its pre-therapeutic increase may provide the persistent cutaneous inflammatory reaction in T-cell-derived skin diseases. Additionally, the post-therapeutic reduction of bcl-2+ cells might represent a key mechanism of medium-dose UVA1 phototherapy.