In this article, authors presented several characteristic features of Gastrointestinal Stromal Tumors (GISTs), which may lead to diagnostic errors and unexpected difficulties during interpretation of CT images. In case of GIST, one may encounter problems at every level of diagnostics and treatment response monitoring, especially after introduction of modern molecular targeting therapies. RECIST scale criteria, used until now for evaluation of response to treatment in solid tumors are based merely on the size of the tumor and turned out to be inadequate after introduction of treatment with new molecular target drugs such as Imatinib.