Abstract. The relationship between in vivo behavior and in vitro characteristics of 59 platelet concentrates (PC) stored for up to 14 days in a synthetic medium or in CPDA-1 plasma was systematically investigated. 25 paired studies (1 study was incomplete) were performed comparing platelets suspended either in the synthetic medium or CPDA-1 plasma with 5 days (n = 5); 7 days (n = 10); 10 days (n = 5); and 14 days (n = 5) of storage. In addition, 10 control studies were performed with freshly prepared PC (6–24 h) in CPDA-1 plasma. Both percent recovery and survival estimations showed decreases with increasing storage duration, irrespective of storage medium used. In both media, with prolonged storage, the platelet survival curves not only became shorter, but also increasingly exponential, suggesting that in vitro storage caused progressive damage to the platelets present in circulation. Survival curves of platelets suspended in synthetic medium remained more linear, indicative of less random damage during storage. Mean population lifespan (MPL) of the stored PC was determined by the area below the survival curve divided by the mean percent recovery for the fresh PC, which was 55%. MPL decreased from 4.5 days (fresh PC) to 0.4 days after 14 days of storage in plasma, with a 50% reduction (t½) estimated at 7.2 days of storage. MPL t½ for PC stored in the synthetic medium was estimated to be 8.8 days. The decrease in in vivo viability with prolonged storage was paralleled with loss of energy-dependent in vitro parameters such as hypotonic shock response, shape change with ADP, and ATP levels, and with increased lactate levels. Although the length of the storage period was shown to be the major factor responsible for the variability in MPL observed in this study, multiple linear-regression analysis showed also that platelet discoid shape, as measured by the shape change, and lactate production were independent in vitro parameters with significant predictability of the in vivo viability. Combined with days of storage in the regression equation, these parameters were found to ‘explain’ 78% of the variability of the MPL found in this study.