The aim of this study was to identify the predictors of malignancy and to demonstrate the uncommon radiological features of gastrointestinal stromal tumours (GIST) using CT. 30 patients participated in this retrospective study. CT findings were 2 evaluated by two radiologists, and categorical variables were compared using chi(2) or Fisher's exact tests. These tests were used to assess any possible relationships between mitotic index and (i) tumour location, size, contour and growth status, (ii) presence of calcifications, necrosis or lymphadenopathy, (iii) enhancement pattern, (iv) metastasis to the liver or other distant sites and (v) invasion of the mesentery or adjacent organs. Tumours were located in the stomach (n=22), small bowel (n=6) and colon (n=2). The size, localization, enhancement pattern, metastasis to the liver or other distant sites and the cystic-necrotic component of the tumours were all associated with a high mitotic index in statistical analyses (p < 0.05). Uncommon radiological findings seen on CT were saccular aneurysmal dilatation, malignant lymphadenopathy and satellite nodules. in conclusion, CT features of GISTs were helpful in identifying predictors of malignant behaviour. (C) 2008 The British Institute of Radiology.