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We conducted a prospective study with a 2-year follow-up.To compare pain, functional limitations, and work status indices as measures of outcome among back pain patients.Work status, pain, and functional limitations indices are often considered as interchangeable outcome measures in back pain research. This perspective has been criticized by several authors, who argue that each of these outcome measures reflects a different construct that may vary independently of the others.The study was conducted on 720 patients, who sought care for back pain in primary care settings of a large health maintenance organization in 1989–90, and were interviewed one month and two years later. Χ2 analyses and receiver operating characteristic curves were used to compare the accuracy of a pain rating and a modified 16-item Roland–Morris score in classifying patients on work status and on the change in work status over time.Moderate agreement between the pain and functional limitations measures and work status was observed. Pain and functional limitations change scores agreed moderately with improvement in work status, but were poorly associated with decline in work status.Although the pain, functional limitations, and work status indices examined in this study are related, they are not equivalent and should not be regarded as interchangeable. These results argue for a clearer distinction of outcome measures in back pain research.