Mechanical obstruction is a major differential diagnosis for dogs presented with gastrointestinal problems. Small intestinal dilation is a cardinal sign of obstruction but its recognition depends upon the observer's experience and anecdotally derived parameters for normal small intestinal diameter. The objective of this study was to formulate a quantitative index for normal intestinal diameter and evaluate its usefulness in predicting small intestinal obstruction. The material consisted of survey abdominal radiographs of 50 normal dogs, 44 cases of intestinal obstruction and 86 patients which subsequently had an upper gastrointestinal examination. A ratio of the maximum small intestinal diameter (SI) and the height of the body of the fifth lumbar vertebra at its narrowest point (L5) was used, and a logistic regression model employed to determine the probability of an obstruction existing with varying degrees of intestinal dilation. A value of 1.6 for SI/L5 is recommended as the upper limit of normal intestinal diameter for clinical use. The model showed that obstruction is very unlikely if the SI/L5 value is less than this. Higher values were significantly associated with obstruction.