A series of observer variation studies in a small group of patients suffering from severe acute ulcerative colitis is reported. Seventy-two separate assessments of the patients' physical signs and clinical progress were made by three independent observers.The results of this investigation suggest that there is difficulty in eliciting general physical signs such as anaemia or dehydration in these patients. By contrast, local abdominal signs such as tenderness and distension were relatively reliably elicited. The results also suggest that there are considerable problems in evaluating these clinical signs in terms of the patient's immediate subsequent progress.