PURPOSE: To develop and test the validity and interrater reliability of the Incontinence-Associated Dermatitis and its Severity Instrument, a novel tool describing incontinence-associated dermatitis and its severity. METHODS: Investigators consulted 2 certified WOC nurse experts to determine face validity of the instrument. Seven additional certified WOC nurses evaluated the tool for content validity. The instrument was tested for interrater reliability by attendees at the 2007 WOCN Society National Conference and employees at a health care system in Minnesota. Criterion validity was determined by comparing IAD severity scores of testers with those of the 2 WOC experts and PI. RESULTS: Consultants determined that the instrument had face and content validity. Two hundred forty-seven attendees at the WOCN Society's National Conference (mean nursing experience 25 ± 9 years [mean ± SD]), 100 nursing staff in Minnesota: 84 staff nurses (mean nursing experience 17 ± 12 years), and 16 nursing assistants (mean nursing experience 7 ± 7 years) determined that the instrument has content validity. There were no significant differences in scores of IAD severity for the 4 cases among the 3 types of testers (P > .05). Overall agreement between the 347 raters and the experts using intraclass correlation was 0.98 (P =.006). CONCLUSION: The Incontinence-Associated Dermatitis and its Severity Instrument demonstrated good validity and interrater reliability and its development fulfills an unmet need in both research and clinical practice settings.