A postoperative handicap index designed to predict diarrhoea and malnutrition following bowel resection in patients with Crohn's disease is proposed. The index takes into account the location and extent of resection, and its value can be calculated from operative records. Retrospective (n = 218) and prospective (n = 68) series of patients were studied. Diarrhoea and malnutrition developed in 102 patients (47 per cent) and 13 patients (6 per cent) respectively in the retrospective series, and in 40 (59 per cent) and one (1 per cent) of those in the prospective series. The handicap index correlated with faecal weight and faecal fat in 112 patients tested. Positive and negative predictive values of an index score greater than 20 for the development of diarrhoea, and over 50 for the development of malnutrition, were 0.64 and 0.90, and 0.60 and 0.99 respectively in the retrospective series; values were 0.80 and 0.71, and 0.25 and 1.00 in the prospective series. The postoperative handicap index is a useful tool for predicting the functional consequences of bowel resection for Crohn's disease.