Adjustment disorder (AjD) is a frequent but under-researched diagnosis due in part to a lack of specific symptom criteria and adequate tools of measurement. The ICD-11 for the first time proposes a positive symptom catalogue to define AjD. This study presents a validation of the Adjustment Disorder - New Module (ADNM), the first symptom severity measure for AjD according to the ICD-11 concept. Validity and sensitivity to change were investigated in a sample of 190 individuals with a DSM-IV diagnosis of AjD. The ADNM scales demonstrated convergent and discriminant validity for anxiety symptoms (Hamilton Anxiety Scale; psychic anxiety r = 0.18-0.31), functional impairment (Sheehan Disability Scale; r = 0.18-0.47), and depression (Montgomery-Asberg Depression Scale; r = 0.13-0.30). At baseline 78% of the individuals with a DSM-IV diagnosis of AjD were also classified so by the ADNM. Repeated-measures ANOVA indicated significant ADNM-symptom decrease during treatment, replicating the patterns of the Hamilton Anxiety Scale, Sheehan Disability Scale, and Clinical Global Impression Scale. This article presents the first use of the ADNM as a measure for ICD-11 AjD in a randomized-controlled intervention study of AjD. It provides support for the construct validity and sensitivity to symptom change of this scale during pharmacological treatment.