Based on the hypothesis that self-determined motivation is associated with an increased likelihood of future behavior, the present study examined the ability of a motivational interview to promote internal motivation for giving blood and future donation attempts. A sample of 484 recent whole-blood and double red blood cell donors (62.4% female; age = 30.2 ± 11.8 years) were randomly assigned to either a telephone-delivered motivational interview or a control call approximately 6 weeks after donating. Several weeks before the call and again 1 week after the call, participants completed the Blood Donor Identity Survey, a multidimensional measure of donor motivation, to derive indices of amotivation, external motivation, and internal motivation to give blood. Repeat donation attempts were tracked using blood center records. Relative to controls, participants in the motivational interview group showed a shift toward more self-determined motivation, as indicated by significant decreases in amotivation (p = 0.01) and significant increases in external (p = 0.009) and internal (p = 0.002) motivation. Furthermore, those with initially high levels of autonomous motivation were more likely to make a donation attempt in the subsequent year if they completed the motivational interview (71.1%) versus the control call (55.1%). Motivational interviewing is a potentially useful strategy to enhance retention of existing blood donors, particularly among those who express a greater sense of internal motivation for giving.