The Glasgow Composite Measure Pain Scale was developed to measure acute pain in dogs in a hospital setting. In this investigation a modified version of the scale was applied in a centre with a different surgical case load and analgesic protocols, and where English is not the first language, to test its validity in a different clinical environment. The modified scale was used to score pain in 60 dogs during the 24 hours after surgery. Their levels of sedation and a clinical impression of their pain were scored at the same time. Three questions were considered; first, how the modified pain score was related to the pain assessed subjectively, secondly, how it related to variables such as the surgical procedure and the dog's health and thirdly, how it changed over time. The mean modified pain scores for the dogs rated subjectively as having no, mild, moderate or severe pain were significantly different, indicating that the modified scale distinguished between pain of different severities. The changes in the dogs' scores also followed the expected changes in their level of pain with time, providing empirical evidence that the scale measures pain.