The aims of the bioptical diagnosis of colitis are delineated in introductory remarks. Subsequently, the range of normality is described for mucosal structures, and the term "nonspecific chronic colitis" is critically discussed. Furthermore, some characteristic (diagnostic) forms of colitis are presented together with their differential diagnoses. Also the "less" diagnostic features of infectious colitis and chronic inflammatory bowel disease are described. An algorithmus of the differential diagnosis of colitis is proposed. Excluding 83 cases of normal findings, the practical impact of such an algorithm was critically analysed in 381 consecutive biopsies of histologically proven colitis.--Several "characteristical" types of colitis could be distinguished in about 95% of the biopsies; in a small percentage of cases only a differential diagnosis could be offered, partly because of inconclusive morphological findings, partly because clinical data were completely lacking and partly because there were no separate rectal biopsies in these cases. We wish to stress, however, that the anamnestical data relating to the length of complaints and the therapeutic interventions are very relevant to the interpretation of the morphological differential diagnosis of chronic colitis, especially in chronic colitis with low activity or inactive disease.