Background: Donor retention is essential for blood banks because acquiring new donors is more expensive than retaining existing ones. Previous studies show that the temporary deferral of donors negatively impacts future donation likelihood. In this study, we analyze the impact of temporary deferrals on future donation behavior while correcting for potential endogeneity, depending on the level of donor experience and number of previous deferrals. Study Design and Method: We use data from more than 123,000 whole blood donors of the Austrian Red Cross over a period of 5.5 years. We estimate logit models to analyze how a deferral affects future donation behavior while controlling for potential selection biases because donors are not deferred ran domly. We control for gender, blood type, years since first donation, and num ber of previous donations and deferrals. We analyze the direct deferral effect, its interaction with donor experience, and the number of previous deferrals. Results: Our results confirm that temporary deferrals hurt future donation behavior. This effect varies with donor experience and the number of previous deferrals. The effect is weaker with a higher number of previous donations and is stronger with a higher number of previous deferrals. The results suggest that donors learn to cope with deferrals the more they donate. However, the nega tive effect of deferrals amplifies over time, and each additional deferral decreases donation likelihood. Conclusion: Blood banks that seek to overcome the negative effect of deferrals should be aware that this effect varies with donor experience and with the number of previous deferrals. Our results suggest that blood banks should focus on early stage donors who are deferred because the negative deferral effect is stronger for more experienced donors. At the same time, blood banks should be careful with donor groups who have experienced deferrals in the past because every additional deferral demotivates future donation behavior. Overall, researchers should be careful to correct for endogeneity because our results suggest that ignoring these effects could lead to substantial underestimation of the negative deferral effect.