The generation of photoproducts of psoralen (POPs) might be relevant in cell death induced by psoralen plus UVA, namely PUVA, which is a recognized effective treatment for cutaneous T-cell lymphoma, chronic graft-versus-host disease, and psoriasis. We investigated the occurrence of POP-induced cell death and the underlying mechanisms. POPs were produced by irradiating a psoralen solution with UVA. Jurkat cells treated in the dark with these mixtures died mainly through an apoptotic mechanism. POPs were separated by high-performance liquid chromatography (HPLC), and cells were added with each of these fractions. A total of 2 dimers of psoralen and 6-formyl-7-hydroxycoumarin (FHC) were identified in the apoptogenic fractions. Apoptosis was preceded by mitochondrial dysfunction caused by the opening of the mitochondrial permeability transition pore (PTP). In fact, both mitochondrial depolarization and cell death were prevented by the PTP inhibitor cyclosporin A (CsA). PTP opening was also documented in isolated mitochondria added with POP, suggesting that apoptosis is caused by a direct effect of POP on mitochondria. In fact, FHC alone induced PTP opening and CsA-inhibitable cell death of Jurkat cells, whereas nontransformed T lymphocytes were resistant. Along with identifying novel apoptogenic molecules, the present results indicate that POP generation directs transformed cells to apoptosis.