As part of the development of the Eleventh Revision of International Classification of Diseases and Related Health Problems (ICD-11), the World Health Organization Department of Mental Health and Substance Abuse is conducting a series of case-controlled field studies using a new and powerful method to test the application by clinicians of the proposed ICD-11 diagnostic guidelines for mental and behavioural disorders. This article describes the case-controlled field study for Disorders Specifically Associated with Stress. Using a vignette-based experimental methodology, 1,738 international mental health professionals diagnosed standardized cases designed to test key differences between the proposed diagnostic guidelines for ICD-11 and corresponding guidelines for ICD-10. Across eight comparisons, several proposed changes for ICD-11, including the addition of Complex PTSD and Prolonged Grief Disorder, produced significantly improved diagnostic decisions and clearer application of diagnostic guidelines compared to ICD-10. However, several key areas were also identified, such as the description of the diagnostic requirement of re-experiencing in PTSD, in which the guidelines were not consistently applied as intended. These results informed specific revisions to improve the clarity of the proposed ICD-11 diagnostic guidelines. The next step will be to further test these guidelines in clinic-based studies using real patients in relevant settings.