This chapter demonstrates the construction of a validity argument for use of a new test of productive academic grammar test as one source of data for placement of students into academic writing classes. Drawing upon an argument-based approach, warrants and assumptions for the utilization and consequence implication inferences were developed to guide the investigation. Mixed methods were used to investigate hypothetical placement outcomes and impacts of academic grammar test score use on the program, test takers, and ESL writing instructors. Findings supported the assumptions by showing that (a) the value of grammatical writing ability is appreciated by all stakeholder groups; (b) academic grammar test scores can identify false positives identified as qualifying for ESL writing exemption; and (c) both test takers and ESL writing instructors would benefit from the use of academic grammar test scores for ESL writing placement and diagnostic feedback that could be generated. This study demonstrates that prospective test use and consequences can be investigated even at a pilot testing stage, when test developers should identify and minimize potential negative impact on stakeholders to improve the quality of the test and the compose appropriate advice about its use.