The purpose of diagnostic language assessment is to identify learner strengths and weaknesses so that subsequent learning activity can be planned according to learner needs, a purpose which aligns with pedagogical recommendations to individualize instruction. Indeed, the extent to which diagnostic assessment can inform the selection of instructional targets and in turn promote individual linguistic development is critical to validity. In this mixed-methods study, I report on evidence for the use of a new second language Korean pronunciation diagnostic. I first compare the self-assessments and diagnostic scores of 198 learners to consider the potential to beneficially raise awareness of strengths and weaknesses. I supplement these findings with analysis of 21 learners' reactions to score reports. Next, I focus on the learning activity and learning gains of a subset of 14 learners whom I interviewed and retested approximately 3 months after receiving their initial feedback. Results indicated that (a) many learners had gaps in their self-assessments of pronunciation, (b) learners readily understood the meaning and intended purpose of diagnostic information (i.e., to guide learning activity), and (c) learners who sustained application of diagnostic information in their self-directed learning efforts could make measurable improvements to their pronunciation.