It is stated that 10-20% of cases of non-specific inflammatory bowel disease cannot be classified. Thirty such cases, designated colitis indeterminate at the time of colectomy, were identified from the pathology files of St. Mark's Hospital. The Histopathological features of the surgical specimens and any available biopsy specimens were studied. In nearly all the cases urgent surgery had been required and the features of incipient or established fulminating disease were present. The pathology of these cases of Crohn's disease and ulcerative colitis overlapped, and differentiating features were scant or unreliable. Accepted criteria of Crohn's disease--namely, fissuring ulceration, transmural inflammation, and a maintained goblet-cell population--were found in cases subsequently proved to be ulcerative colitis. Disease activity greatly affected the evaluation of morphological features. Many of the difficulties were resolved when biopsy material obtained during a quiescent phase was examined. The specimens gave a dynamic perspective of the disease process, often more valuable than the static, non-specific picture of acute disease seen in the surgical specimens. Case of colitis indeterminate form a small distinctive group in the spectrum of inflammatory bowel disease which is characterised by a common pattern of pathology that presents a diagnostic dilemma.