BACKGROUND: Continuous effort around the globe is exerted into donor management, with the overall aim of keeping a constant flow of new donations. To achieve this goal, blood banks and blood donor associations are taking initiatives to get the same donor to make repeated donations. In general, such initiatives are implemented without proper evaluation. This study circumvents this trend and evaluates the effect on donations from sending blood donors a text message when their donation has helped a patient. METHODS AND MATERIALS: In a large-scale field experiment (n = 20,365), we randomized donors such that half are sent a text message when their donation was used. Among those who were randomized to the text message, we also randomized the time of day the text message was sent. RESULTS: The messages increased subsequent donations by 3.6% (p = 0.023), compared with donors who did not get the message. It was also shown that optimizing the time of day when the text messages were sent increased the effect substantially. Text messages sent in the evening increased donations by 6.5% (p = 0.011). Finally, it was found that, among whole blood donors, the text messages did not have a significant effect (p = 0.554), whereas it did among the plasma donors (p = 0.003). CONCLUSION: Several blood banks have adopted the strategy of sending blood donors a text message that acknowledges that donors' contributions helped a patient. This study is the first to formally test the effects of such a message with a control group.