Eleven laboratory parameters were compared with the clinical grading of disease activity in 44 patients with ulcerative colitis, using multiple stepwise regression analysis to identify which measurements significantly reflect the severity of the disease. Only 2 parameters, serum orosomucoid and haematocrit showed significant correlation with clinical status. It is suggested that in the method of assessing disease activity proposed by Truelove and Witts the haemoglobin, which in this study was found to have no value, could be replaced by an equally simple test, the haematocrit. Comparisons are made between the relative usefulness of these parameters in ulcerative colitis and Crohn’s disease.