Background aims. The clinical benefi ts of extracorporeal photochemotherapy (ECP) are well recognized, but its clinical use is limited by logistical diffi culties, especially because of the need to perform repeated aphereses. The cryopreservation of mononuclear cells could allow maintenance of the ECP schedule while reducing the number of aphereses. The aim of this work was to assess whether previous cryopreservation impairs the immunomodulatory function of ECP-treated peripheral blood mononuclear cells (PBMC). Methods. Fresh or previously cryopreserved PBMC were exposed to ECP and added on day 0 into a mixed leukocyte reaction. Proliferation of alloreactive lymphocytes was measured by carboxyfl uorescein succinimidyl ester (CFSE) dye dilution. Apoptosis was quantifi ed by annexin-7AAD staining. Results. ECP-induced apop-tosis was slightly increased in cryopreserved cells but the kinetics of apoptosis were similar to fresh cells. Lymphocytes stimulated in the presence of ECP-treated PBMC displayed a signifi cant decrease in proliferation. The suppression was enforced when ECP-treated cells had been activated previously by allogeneic stimulation. Cryopreservation before ECP exposure did not impact apoptosis triggering or anti-proliferative properties of ECP-treated cells. Conclusions. Cryopreser-vation before ECP does not impair the immunomodulatory effects of treated cells. These data warrant investigation of the clinical use of cryopreserved PBMC for ECP.