The report presents 200 cases of gastrointestinal stromal tumors (GIST). The material originated from six diagnostic centers in Poland and was reclassified according to the current criteria. Among lesions other than GISTs, 14 were identified as smooth muscle tumors and seven as neural tumors. GISTs were located in the stomach (51-63.3% of the investigated series), small intestine (27.4-33.8%), colon (approximately 4.5%), abdominal cavity, i.e. in the peritoneum and omentum (6%), and in the retroperitoneal space (2.5%). A slight predominance of women was noted (53-56%). The age of the patients ranged between 14 and 93 years of life, with the mean age of 62.4 years. Individuals younger than 45 years of age accounted for 10% of the group. In ten patients (five of them less than 45 years of life), multiple tumors were detected, their number ranging from two to less than 20; these individuals constituted 5% of the entire series. Moderately and highly aggressive tumors predominated. In the series, when multiple tumors were excluded, a total of 24 epithelioid GISTs (12%) were observed; of this number, 13 were situated in the stomach, six--in the small intestine, two--in the abdominal cavity and another two in the retroperitoneal space. Synchronic tumors observed in patients with GISTs were seen in seven patients, including an adenocarcinoma of the colon, two adenocarcinomas of the stomach, a carcinoid tumor of the small intestine, a pheochromocytoma of the retroperitoneal space, an anaplastic lymphoma and a disseminated squamous cell carcinoma. In immunohistochemical reactions (CD117, CD34, SMA, S-100, DES), attention was focused on the immunoreactivity of small GISTs, below 2 cm in size, and of multiple tumors. Immunohistochemical reactions were equally differentiated as to their presence and intensity in small tumors and in highly aggressive lesions above 5-10 cm in size. In multiple GISTs, immunohistochemical tests strongly indicated the heterogeneity of neoplastic cells, which, nevertheless, showed no consistent association with the location of the tumor, its aggressiveness, cellular structure or a tendency to form multiple foci.