The incontinence severity index (ISI) consists of two questions, regarding frequency and amount of leakage. It categorizes urinary incontinence (UI) into slight, moderate, severe, and very severe. The purpose of this study was to test its validity. The index was compared with the results of pad-weighing tests performed by 200 incontinent women referred to a hospital clinic and 103 at a primary care incontinence clinic. Inconvenience was scored by a six-level Likert scale. Mean pad-weighing results (grams per 24 hours, 95% confidence intervals) were 7 (4-10) for slight, 39 (26-51) for moderate, 102 (75-128) for severe, and 200 (131-268) for very severe UI. Spearman's correlation coefficient for pad-weighing results and severity index was 0.58 (p<0.01), and inconvenience increased significantly with increasing severity. The ISI demonstrated good criterion validity against 24-h pad tests. Good construct validity was indicated by a clear link between ISI and inconvenience.