Background. Pathogen reduction in platelet concentrates (PC) using Amotosalen/UVA-light reduces the risk of transfusion transmitted infections but also decreases the post-transfusion platelet count increment. Little is known about potential platelet lesions caused by Amotosalen/ UVA-light, which may reduce therapeutic efficacy of PCs. Methods. Platelets from buffy coat (n=15) derived PCs were pooled and split into three equal PC-units. PC-1 was left untreated (control). PC-2 was Amotosalen/UVA treated using the INTERCEPT Blood System™, PC-3 was gamma irradiated (30 Gy). Samples were prepared one and five days after PC-production for LC-ESI-MS/MS analysis. Proteins displaying treatment-dependent changes in intensity were classified according to Gene Ontology. Results. In total, 948 proteins were identified, 721 with ≥2 peptides. At day 1, Amotosalen/ UVA-treatment triggered alteration of 23 proteins, and gamma irradiation of 49 proteins (overlap: 11 proteins). Five days storage revealed 58 (Amotosalen/UVA treated), 50 (gamma irradiated), and 36 (controls) changes in the platelet proteome compared to control platelets at day 1. Gene Ontology analysis revealed that many affected proteins were displaying specific catalytic activities and/or protein/nucleic acid binding capacity. We identified platelet endothelial aggregation receptor 1 precursor, chloride intracellular channel protein 4, and protein-tyrosine sulfotransferase 2 as proteins uniquely and consistently altered after treatment and storage of Amotosalen/UVA treated platelets. Conclusion. While Amotosalen/UVA-treatment causes less pronounced proteome changes than gamma irradiation at day 1, our data indicate an increase in storage lesions at day 5 caused by this pathogen reduction treatment.