The amount of acid citrate dextrose formula A (ACD-A), which is a commonly used anticoagulant in leukopheresis, has to ensure both the safety of the donor and guarantee the integrity of the peripheral blood stem cell (PBSC) product until its transplant. Two recent consecutive cases of postthaw PBSC product clotting initiated a look-back investigation of the ACD-A percentage in leukopheresis products collected in our facility. The data indicated a significant difference between the average amount of ACD-A in prefreezing products collected during 2006 (11.4%) and in products collected during 2007 and 2008 (8.8% and 8.7%, respectively). These findings and the fact that the two clotted products had less than 7% ACD-A indicated that insufficient amount of anticoagulant might contribute to their clotting. This investigation prompted us to modify our collection and thawing procedures to prevent similar events in the future.