This article describes findings from an evaluation of the National Science Foundation’s (NSF) International Research Fellowship Program (IRFP), which supports postdoctoral research fellowships of 9–24 months in locations outside the USA. The evaluation assessed the role of IRFP in seeding productive international research collaborations for researchers early in their careers. Data used in the evaluation included the NSF program data and survey data from 1,039 scholars who applied to IRFP from 1992 to 2009. The evaluation employed a quasi-experimental design to compare the outcomes of IRFP-funded postdoctoral fellows (awardees) to unfunded applicants (non-awardees), using pre-award characteristics of applicants to mitigate the potential threat of selection bias; the study incorporated propensity score methodology to construct groups of awardees and non-awardees that were statistically similar across a number of preexisting characteristics. By constructing a comparison group from among applicants to the program, the treatment and comparison groups represented individuals who were similarly motivated to engage in international collaboration, and by using propensity scores to match based on characteristics at the time of application, comparisons were made among similarly qualified groups. The study found that IRFP awardees were more likely to have productive research collaborations with foreign researchers, their time abroad did not come at the expense of overall research productivity or career advancement, and the fellowships seeded collaborative relationships that extended beyond the fellowship period.