Nursing advocacy has been defined in several ways, but its structure has not been operationalized. Therefore, different theoretical aspects of advocacy have not been able to be verified empirically. Our aim was to validate the content and structure of the concept of nursing advocacy within procedural pain care from the point of view of both patients and nurses. The concept of nursing advocacy was outlined first by integrative literature review and then delineated with data of semi-structured interviews among adult internal medicine and surgical patients and nurses (phases I-II). Then, the instrument was developed with facet-design approach, and the content and construct validity of it established in two subsamples within otolaryngeal surgical care (phases III-V). These descriptive and explorative studies were conducted from 2003 to 2007 in a total of 12 Finnish hospitals. The findings supported the validity of both the concept and the instrument. Pearson correlations and Cronbach alphas supported the conceptual dimensions of advocacy operationalized in this instrument. Internal consistency validity of the instrument scales were supported by subscale CVI, which ranged from 0.97 to 0.99, and item CVI, which ranged from 0.93 to 1.00. The instrument structure was strengthened with exploratory factor analysis, which explained 65% (sample of patients) or 57% (sample of nurses) of the variance in antecedents, 75% or 74% of the variance in activities, and 60% or 56% of the variance in the consequences of advocacy. This four-part 56-item (58-item for nurses) instrument promises to measure the dimensions of nursing advocacy. However, the instrument needs further validation in different settings.