question: BACKGROUND Plasma contains many important proteins of therapeutic interest including albumin, clotting factors, and antibodies. Source plasma (SP) is in great demand particularly due to a shortage of immunoglobulin. To better understand how to increase supply, we examined SP donor deferrals for the previous 3 years. STUDY DESIGN This is a description of donor deferrals at 255 plasma donation centers in the United States for April 1, 2017 to March 31, 2020. RESULTS A total of 4 587 923 events were evaluated for the 3-year period 2017-2020. There were 873 227 deferrals analyzed for 2017-2018, 1 765 582 in 2018-2019, and 1 949 114 for 2019-2020. The most common deferral each year was for unacceptable blood pressure (BP) or pulse which comprised 27.9%, 28.2%, and 28.3% of deferrals in 2017-2018, 2018-2019, and 2019-2020, respectively. The second most common cause of deferral was for unacceptable hematocrit which comprised 14.1% of deferrals in 2017-2018, and 16.0% in 2018-2019 and 2019-2020. The majority of these deferred donors had low hematocrits and were predominately (~80%) female. Deferral for unacceptable total protein comprised a smaller percentage (~4%) of deferrals. DISCUSSION Most donor deferrals were due to unacceptable screening results, particularly high BP, elevated pulse, low protein, and low hematocrit. Although rates of deferrals in other categories have been slightly increasing over time, they comprise a small percentage. Donor education regarding healthy lifestyle choices may improve overall donor health, decrease deferrals, and increase SP supply.