Background Imaging with F18-fluorodeoxyglucose PET/CT is used to determine sites of abnormal glucose metabolism and can be used to characterize and localize many types of tumors. Aim To assess the prevalence of multiple primary malignant neoplasms (MPMN) detected by PET/CT in cancer patients. Material and Methods F18-fluorodeoxyglucose PET/CT scans performed to 800 patients with a newly diagnosed cancer or with already treated tumors were retrospectively reviewed. In patients whose examination described incidental findings not related to the primary tumor, a research was done about further laboratory, imaging or pathological studies. Results In 188 PET/CT scans (23%) an incidental finding was found. Of these, 66 (35%) were considered as MPMN, 12 as atypical metastases of a known primary tumor, 14 as false positive images (inflammatory or physiologic uptake) and 29 as benign or low grade tumors. In 67 cases (36% of all incidental tumors), the finding was not confirmed. Seven percent of patients with a newly diagnosed tumor had a synchronic MPMN detected by PET/CT. Nine percent of patients with treated tumors developed a metachronous MPMN during their follow up. The most common incidental tumors were thyroid cancer in 15 cases, kidney cancer in 13, lung cancer in 10, colorectal carcinoma in 9, breast cancer in 6, prostate cancer in 4, non-Hodgkin lymphoma in 3 and pancreatic cancer in 2. Conclusions A MPMN is detected by PET/CT in a significant number of cancer patients.