An instrument for measuring function-related quality of life (QOL) in patients with ulcerative colitis was tested for validity, reliability, and responsiveness, using concepts and statistical methods easily understandable for pharmaceutical researchers. For this 8-week study, 374 patients were randomised and received placebo or oral mesalazine (mesalamine) at 1g, 2g, or 4g daily. A 10cm visual analogue scale and patient diaries were used to measure the 12 QOL parameters at baseline and study termination. Physician's Global Assessment was measured at end-point and used to assess change in the disease state of each patient. Analysis of covariance was used to test the construct validity, reliability and responsiveness of the instrument. Construct validity (p less than or equal to 0.0001), reliability (p greater than 0.05), and responsiveness (p less than or equal to 0.0001) were established for all 12 parameters of the instrument. We conclude that the instrument demonstrated precision in measuring function-related QOL in patients with active ulcerative colitis. Furthermore, content validity was maximised by combining disease-specific and general questions in the instrument.