BACKGROUND Platelets (PLTs) have been associated with the highest rate of transfusion-associated adverse events (AEs) among all blood products. Most of PLT-associated AEs are considered to have an inflammatory mechanism. However, it is still unclear whether prolonged storage of platelets is associated with an increased rate of transfusion-related AEs, especially in the era of universal prestorage leucoreduction. METHODS In this retrospective study, 52 649 PLT products consisting of about 80% apheresis PLTs and 20% whole blood-derived (WBD) PLTs were transfused to 9415 patients from July 2011 to March 2017. All the PLTs were leucoreduced prior to storage. All but 69 units of the apheresis PLTs were irradiated and none of WBD PLTs were irradiated. During this period, a total of 284 AEs that were reported to the transfusion service were analysed. RESULTS Univariate and multivariate logistic analyses showed that apheresis/irradiated PLTs and PLT age were associated with a significantly increased frequency of inflammation type AEs (OR (95% CI): 2·24 (1·32, 4·15) and 1·30 (1·12, 1·52), respectively). There was a significant increase in the frequency of inflammation AEs associated with prolonged storage of apheresis/irradiated PLTs [OR (95% CI): 1·26 (1·03, 1·53)]. In contrast, there was no association between allergic symptoms and PLT age. Moreover, the frequency of transfusion AEs associated with apheresis/irradiated PLTs (57·2/10 000) was significantly higher than that of WBD/nonirradiated PLTs (26·0/10 000) (P < 0·01). CONCLUSIONS Prolonged storage of apheresis/irradiated PLTs was associated with a higher frequency of inflammation AEs. Apheresis/irradiated PLTs caused more AEs than WBD/nonirradiated PLTs.