The purpose of this study was to evaluate the construct validity, generalizability, and interrater reliability of an instrument designed to measure the complexity of nursing care in the ambulatory care setting. The conceptual framework for the development of the classification instrument was adapted from a theory of organizations and is based upon knowledge technology. Previous research using the Delphi methodology delineated 44 activity categories for the classification instrument. These categories were organized under six literature-based responsibility areas. The instrument was tested in seven areas of one hospital-based ambulatory care setting. A total of 641 valid patient ratings were used for the analysis. Results indicate the instrument has beginning construct validity and high interrater reliability with training. It is not generalizable across services unless the effect of clinic type on nursing care complexity is considered.