Background: Legitimate concerns exist over the safety of donors during multicomponent apheresis collections (MACs), when large volumes of red blood cells (RBCs) and plasma are removed. This study evaluates the predictive value of various donor- and procedure-related variables for moderate to severe donor acute adverse events (AAEs)., Study Design and Methods: Data on all apheresis donation procedures performed at a large university hospital-based donor center over a 2-year period were obtained by a review of adverse event forms and procedure logs (Trima Accel 5.1, Gambro BCT). Various donor- and procedure-related variables were compared between procedures that resulted in moderate to severe AAEs and those that did not., Results: Moderate to severe AAEs occurred in 53 (0.47%) of 11,333 apheresis donation procedures. The majority of events (96.2%) had predominantly features of vasovagal reactions (VVRs). Females were at significantly higher risk (odds ratio [OR] = 2.8, p < 0.0003) compared to males. Donors who experienced AAEs had significantly lower predonation total blood volume (TBV) and hematocrit (Hct) and higher total RBC loss and net fluid loss at the end of the procedures. Total plasma loss alone was not significantly different between the two groups. Total blood loss was significantly higher among donors who experienced AAEs as a percentage of the donor's TBV., Conclusion: Apheresis collections are well tolerated even when multiple components are collected, with a very low overall incidence of moderate to severe AAEs (0.47%). Small, female donors with lower predonation Hct are at higher risk, especially when RBCs are collected.