The paper analyzes the experience in treating and following up 26 patients who had three or more polyneoplasias with one of them being located in the stomach or large intestine. The stomach, large intestine, and both were involved in 4, 20, and 2 patients, respectively. Tumors of three sites were found in 21 patients, those of four and six sites were in 4 and 1 patients, respectively; 26 tumors were synchronous and 59 were metachronous. Radical treatment was performed in 65 (76%) cases. Surgical and combination treatments were most frequently used. During a follow-up of patients previously treated for malignancies, special attention should be given to the diagnosis of metachronous tumors that are more likely to have previous sites, by keeping in mind the patterns revealed by us and other investigators. The authors concluded that the development of polyneoplasias is a common and steadily growing trend; their presence is not a contraindication for radical, as well as combination, treatment for each of the neoplasms; identification of early-stage tumors and their successful management requires the meticulous medical examination of patients in specialized health care facilities, by taking into account the data on the principal sites of secondary tumors; this patient category needs genetic tests to rule out hereditary syndromes.