For decades, methylene blue has been used in gastrointestinal endoscopy as an absorbing dye, it was, however, not approved for that purpose and has now been withdrawn from the market. A possible substitute is toluidine blue, an acidophilic, metachromatic dye that selectively stains cell nuclei; accordingly, since 2007, toluidine blue has been approved as a topical diagnostic agent in chromoendoscopy. Cells with increased DNA synthesis are stained more intensively so that not only malignant cells but also erosions, ulcerations and inflammatory areas are stained with toluidine blue because of the increased reparative cellular processes. Up to now, absolutely no studies have been carried out with regard to the effectiveness of toluidine blue in gastrointestinal endoscopy. We report on a consecutive series of 364 endoscopic applications of toluidine made on the basis of various indications. Besides the known indications (e. g., chromoendoscopy in case of Barrett's oesophagus), we mostly used toluidine blue, diluted in hydroxyethylstarch (HAES), for submucosal injections of flat adenomas prior to endoscopic mucosal resection or endoscopic submucosal dissection, in order to precisely determine the extension of visible lesions. Local and systemic adverse reactions have not been observed. A demarcation of the lesions can be made as effectively as with methylene blue.