Gastrointestinal stromal tumours (GIST) occur mostly as sporadic solitary 
lesions involving the tubular GI tract and are only rarely associated with other 
benign or malignant neoplasms or occur as part of a multi-neoplastic disease as 
in the setting of Carney's triad and von Recklinghausen's disease. We analysed a 
total of 97 cases of surgically resected GIST looking for various types of 
associated non-GIST malignancies. 18 cases (18.6 %) were identified. There were 
12 women and 6 men aged 43 to 87, average age 71.7 years. 12 GISTs were located 
in the stomach, four in the small intestine, and one each in the duodenum and 
the vermiform appendix. Associated malignancies (mostly carcinomas) were 
gastrointestinal/pancreatic (9), gynaecological (3), mammary (2), renal (1), 
prostatic (1), pulmonary (2) and haematolymphoid (2) in origin. Most GISTs 
(16/18) represented benign or low-risk lesions (innocent bystanders) detected 
during evaluation for the known cancer, either during staging, intra-operatively 
or on follow-up. Two women (43 and 72 years old) with large malignant GISTs 
(10.5 and 12 cm), one of them with two simultaneous hepatic metastases resected 
at the same time as the primary GIST, developed infiltrating ductal mammary 
carcinoma and were alive and well 75 and 95 months postoperatively, 
respectively. Furthermore, one of them developed endometrial carcinoma. We 
concluded that GISTs are not uncommonly encountered in cancer-patients during 
staging, intraoperatively or on follow-up and should be considered in the 
differential diagnosis of newly detected focal lesions to avoid their 
misinterpretation as metastasis from the known malignancy with consequently 
false therapeutic decisions.