Diagnosticians who recommend educational accommodations for postsecondary students with learning, cognitive, and psychiatric disabilities often reference specific diagnostic test scores as a basis for the recommended accommodations. Moreover, accommodation decision-makers often follow diagnosticians’ lead and/or rely on the diagnostic scores themselves to make and justify accommodation determinations. The present paper considers the ecological validity of these diagnostic test scores, focusing on their generalizability across time, setting, and dimension of performance. A wide variety of research suggests a need for circumspection and care when using diagnostic test scores to make accommodation decisions. Illustrative data are presented showing that scores from diagnostic cognitive tests do not significantly predict students’ ability to access a realistic test. Diagnostic tests should not be unduly criticized, and data from these tests can be helpful, but both clinicians and accommodation decision-makers should carefully consider issues of diagnostic test scores’ ecological validity and make nuanced conclusions about the needs of a client/applicant based on a wide variety of evidence.