A composite scale for assessing pain in dogs in a hospital setting has been developed on the basis of observations of their behaviour. Initially, 279 words and expressions suggested by 69 veterinary surgeons were reduced into 47 words and expressions which were allocated into seven behaviour categories: demeanour and response to people, posture, mobility, activity, response to touch, attention to painful area and vocalisation. Three statistical methods, hierarchical agglomerative cluster analysis, Cronbach's alpha coefficient, and analysis of variance with multiple comparisons and empirical cumulative distributions, were used to validate these procedures, and a questionnaire accompanied by a list of definitions was designed around the expressions. The new composite scale is more detailed than previously reported scales for assessing pain in dogs on the basis of their behaviour, and the methods used in its development are based on sound scientific principles.