In the study of work-related stress among nurses, the Nursing Stress Scale (NSS) is the best known and most widely used scale. This article presents an overview of the NSS and its use, and describes the development of an expanded instrument (ENSS) to measure sources and frequency of stress perceived by nurses. Findings are based on a random sample of 2,280 nurses in Ontario working in a wide range of work settings. Pretests for the study indicated that an expanded version of the NSS was necessary in order to adequately measure sources of stress among nurses. The sources of stress comprised nine subscales--death and dying, conflict with physicians, inadequate preparation, problems with peers, problems with supervisors, workload, uncertainty concerning treatment, patients and their families, and discrimination. Confirmatory factor analyses, run on two randomly selected halves of the sample, came close to meeting standard criteria levels. The alpha coefficients of eight of the subscales were .70 or higher, and concurrent and construct validity assessments provided strong support for the expanded NSS.