Codes of ethics of nursing, social work, and medicine, as well as Joint Commission Accreditation Standards, require members of these professions to engage in advocacy on behalf of patients. With use of expert panels, seven categories of patient problems in the healthcare milieu were identified: ethical rights, quality care, preventive care, culturally competent care, affordable/accessible care, mental health care, and care linked to patients' homes and communities. To measure the frequency with which healthcare professionals engage in patient advocacy related to these specific problems, the Patient Advocacy Engagement Scale (Patient-AES) scale was developed and validated through analysis of responses of 297 professionals (94 social workers, 97 nurses, and 104 medical residents) recruited from the personnel rosters of eight acute-care hospitals in Los Angeles County. Hospitals included public, not-for-profit, HMO, and church-affiliated hospitals that served general hospital populations, veterans, cancer patients, and children. Results supported the validity of both the concept and the instrument. Construct validity was supported by testing the hypothesized seven-factor solution through confirmatory factor analysis; 26 items loaded onto seven components. Pearson correlations for the overall scale and seven subscales in two administrations supported their test–retest stability. Cronbach α ranged from .55 to .94 for the seven subscales and .95 for the overall Patient-AES. The Patient-AES is, to our knowledge, the first scale that measures patient advocacy engagement by healthcare professionals in acute-care settings related to a broad range of specific patient problems. © 2014 Wiley Periodicals, Inc.