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Objective—To establish interval level measurement in a prototype composite measure pain scale (CMPS) for assessment of acute pain in dogs and to investigate the scale's validity. Animals—20 clinically normal dogs, 20 dogs with medical conditions, and 117 dogs undergoing surgery. Procedure—First, a scaling model was applied to the CMPS descriptors to establish weights for each and create a continuous scale. Subsequently, 5 observers independently used the scale to score signs of pain in 4 groups of dogs (control dogs, dogs with medical conditions, and 40 dogs undergoing soft tissue or orthopedic surgery). Scores from each group and from groups of conditions perceived to cause no, mild, moderate, and severe pain were compared. In addition, the scale was applied to 77 dogs undergoing orthopedic or soft tissue surgery and scores were compared with simultaneously derived numeric rating scale (NRS) scores; comparisons were made between surgical groups and with time after surgery. Results—Calculated scale descriptor weights ranged from –2.0 to 2.0 and were transformed to create a continuous scale from 0 to 10. Median CMPS scores differed significantly among the 4 study groups and among pain severity groups and were typically greater with increasing perceived pain severity. Agreement was determined between CMPS and NRS scores, and there was a significant and expected time effect and difference between the CMPS scores of dogs undergoing orthopedic and soft tissue surgery. Conclusions and Clinical Relevance—Results indicate that this interval level measurement scale is a valid measure of acute pain in dogs. (Am J Vet Res 2005;66:2154–2166)