Self-efficacy shares a causal relationship with performance. Few studies have examined self-efficacy in relationship to athletic training and instead limit themselves to areas focused on the musculoskeletal system. The purpose of this study was to develop and validate a scale for measuring athletic trainer self-efficacy in the assessment and diagnosis of medical conditions and illness. A principal component analysis yielded 19 items that loaded on three factors. The analysis resulted in acceptable levels of correlation (KMO = .93; Bartlett x2 = 2152.58, df = 171, p < .001) and internal consistency (α = .943). This scale demonstrates both validity and reliability.