During the last decade, the trend toward intensifying chemotherapeutic regimens in patients with hematologic malignancies rapidly increased the demand for single-donor platelet concentrates (PCs). The logistics of such supply, however, necessitated the storage of these blood components prior to transfusion. Today, most blood centers use di(2-ethylhexyl)phthalate-free blood bags, which are assumed to allow a storage period of up to 5 days. This report describes biochemical and functional changes of stored single-donor PCs, which may influence the expected quality of PCs. The acid-base status is characterized by an initial respiratory alkalosis compensated by a metabolic acidosis. Changes in extracellular electrolyte, lactate dehydrogenase, glucose, lactate, elastase, and complement levels, as well as in the release of alpha granule content and the initial activation of plasma coagulation, are demonstrated. These changes result in a functional impairment of stored PCs as reflected by thromboxane and serotonin release reaction and by aggregation and in vitro bleeding time studies. In contrast, in vivo recovery and survival rates have been reported to be unaffected. Whether the good recovery and survival rates are caused by a rejuvenescence of stored PCs in vivo or are due to injured circulating platelets has not yet been proven.