Citrated platelet-rich plasma (PRP) was kept at cold temperatures or room temperature. After 4 hr or more at these temperatures, the PRPs were warmed 1 hr at 37°C. This prevents the spontaneous aggregation seen in chilled PRP that is stirred immediately after warming. Platelet aggregation in response to connective tissue (CT), epinephrine, and adenosine diphosphate (ADP) was considerably greater in the PRPs originally kept at cold temperatures. In addition, chilling would restore the aggregation of platelets whose function had deteriorated due to prolonged storage at warm temperatures. Neither ADP-induced refractoriness, serotonin uptake, or CT-induced serotonin release was affected by cold. Retention in glass bead columns was greater in platelets that had been chilled than in platelets kept at room temperature or 37°C. Thus, the storage of platelets at cold temperatures leads to changes that improve platelet aggregation but may also increase platelet adhesion, which would account for the decreased in vivo survival of platelets preserved for transfusion at cold temperatures.