New opinions on gastrointestinal stromal tumours (GIST) are discussed. Most tumours listed under this classification consist of elongated cells. GISTs are most often seen in the stomach (60%), small intestine (30%), and in other parts of the gastrointestinal tract (10%). Immunohistology staining shows c-kit (CD117) and CD34 positivity. On the other hand, the occurrence rate of stomach leiomyomas is relatively low when compared to that of the oesophagus. Indeed, leiomyomas are seen in the oesophagus quite often. By immunophenotypisation these tumours are c- kit and CD34 negative but show a positive reaction when using antibodies against smooth muscle actin and desmin. Marker CD34 seems to be less sensitive than c-kit (CD117). The second group of tumours are gastrointestinal autonomic nerve tumours (GANTs) which are considered to be a subtype of GISTs. It is quite probable that these are derived from the myenteric nerve plexus. Their histologic appearance is similar to GISTs, however, both have a different electron microscope picture and a different immunophenotype. The similarity of GISTs and Cajal‘s interstitial cells (ICC), present in the wall of the gastrointestinal tube, led to the opinion of their histogenetic similarity. This hypothesis was impaired by findings of similar tumours of the same immunophenotype in the omentum and mesenterium. In these areas the Cajal‘s cells were not present. Tumours of ICC phenotype or of smooth muscle cells are probably derived from primitive stem cells. The estimation of biological activity of GISTs according to their histological picture or immunophenotype is difficult. In doing this, various criteria are taken into consideration. One of the most important criteria is the size of the tumour and its mitotic rate. According to most recent recomendation, the tumours are divided into four categories: GISTs with a very small risk, GISTs with a small risk (previously called benign), GISTs associated with an intermediate risk, and GISTs with a high risk (previously marked as malignant). GANTs generally have a worse prognosis than other GISTs and are considered to be more malignant tumours.

        Key words: gastrointestinal tumours – immunohistology – biological acticity of the tumours – tumour classification