Background: The differential diagnosis between idiopathic proctocolitis (ulcerative colitis) and Crohn’s disease of the large intestine based on clinical and morphological signs sometimes fails. For those instances when the disease cannot be classified the term indeterminate colitis was proposed. Views on the way of differentiation of inflammatory bowel diseases and indeterminate colitis differ and elucidation of views is important not only from the theoretical but also the practical aspect, in particular for surgical treatment. Method: The authors revised retrospectively a group of 1328 patients followed up in 1942–1984 on account of inflammatory bowel diseases with regard to the difficulties associated with the differential diagnosis of idiopathic proctocolitis and Crohn’s disease. For differentiation they used a diagnostic approach where they differentiated corroborating indispensable and excluding signs and on selected examples they demonstrate how they proceeded. The authors present also examples of diseases which simulated inflammatory bowel diseases and where it proved later possible to detect their nature. Results: The most frequent causes of uncertainty were the following ones: a short observation period (in acute cases), a controversy between the clinical picture and the pathological finding and atypical symptoms. It proved possible to eliminate the majority of problems on the basis of the evaluation of the character of the signs and communication between the clinician and pathologist. The whole group included only 19 cases which could not be classified (1.4% of 1328 cases). Conclusion: It proved possible to classify the majority of the investigated patients by differentiation of indispensable, confirming and excluding signs. Differential diagnostic difficulties were encoutered in particular in acute cases where the pathological finding was „unclassifiable“ and where one of the main diagnostic supports was lacking – i.e. knowledge of the course and development of the disease. Such cases have been usually described as „indeterminate colitis“. In our view this term does not imply that a special type of intestinal inflammation is involved. During the subsequent follow up the diagnosis is frequently elucidated in the sense of transient colitis (self limiting), idiopathic proctocolitis, Crohn’s disease and only some of the cases remain indeterminate. In our opinion however no special pathological entity is involved and the description of ulcerative inflammation of the large intestine of unelucidated nature is more pertinent (colitis ulcerosa in the morphological sense). According to the two-component hypothesis it is due to non-specific infection superimposed on a primary disorder of various nature.

        Key words: Crohn’s disease – ulcerative colitis – differential diagnosis – indeterminate colitis