Background: Seeking treatment in a timely manner may prevent lower limb amputation subsequent to diabetic foot ulcers. Knowledge regarding the warning signs is one important factor that is related to the timely treatment seeking of patients with diabetic foot ulcers. However, there is no previous research on which aspects of such knowledge that patients with diabetes are lacking. Purpose: The aims of this study were to test the psychometric properties of a questionnaire that was designed to assess the knowledge of the warning signs of diabetic foot ulcer deterioration and to determine the knowledge of these warning signs among patients with a history of diabetic foot ulcers. Methods: A cross-sectional study design was used. Patientswith a history of diabetic foot ulcers were recruited using convenience sampling (N = 385). Data were analyzed using Spearman’s rank correlation and the MannYWhitney U test. Results: The questionnaire showed good known-group validity, satisfied internal consistency (Kuder-Richardson Formula 20 = .82), and exhibited acceptable 2-week testYretest reliability (Spearman’s rho coefficient = .75). More than 30% of the participants were unaware of the warning signs of peripheral vascular insufficiency and severe infection. Of the participants, 75.8% felt that the time to seek medical treatment of diabetic foot ulcers was when they experienced wound deterioration. Those who held this opinion had significantly less knowledge regarding the warning signs of diabetic foot ulcer deterioration than did thosewho had not.Other factors that were significantly associated with less knowledge included being older, having less education, not having an existing foot ulcer, having no foot ulcer treatment history, and never having received education regarding foot ulcers. Conclusions/Implications for Practice: The questionnaire showed adequate validity and reliability and thus may be used by healthcare practitioners and researchers to assess the knowledge of at-risk patients regarding the warning signs of diabetic foot ulcer deterioration. Patient education regarding the importance of timely treatment seeking and warning signs should be promoted.