Recent advances in biopsy techniques using fibreoptic instruments have refocussed attention on the condition of non-specific duodenitis. Investigation of the topography of duodenitis and its relationship to possible aetiological factors and to duodenal ulcer calls for a proper histological classification. Such a classification is proposed based on the subjective appearances and a quantitative examination of 747 fibreoptic biopsy specimens in a wide range of clinical conditions.