Objective To assess frequency, clinical indications, and cumulative effective dose (CED) of patients undergoing multiple F-18-FDG PET/CT imaging. Methods Retrospective analysis of F-18-FDG PET/CT scans performed at a university hospital for 11 years was done. The effective dose was estimated from activity administered and dose-length-product. Results A total of 55,424 F-18-FDG PET/CT scans were performed in 32,658 patients. The average injected activity was 421 MBq and median 417 MBq. 24.2% of the patients were scanned 2-5 times in a year, 16.7% of them being unique patients (not counted as a separate patient in different years). The maximum PET/CT scans in a year for any patient was five. 23,571 (72.2%) patients underwent a single (18) F-FDG PET/CT scan, while 9087 (27.8%) unique patients underwent 2-23 scans during 11 years. 82% of the scanned patients had a malignant disease, and 2.4% of patients with & GE; 2 (18) scans in a year had non-malignant indications. 1.4% of patients received CED & GE; 100 mSv in one year from multiple (18) F-FDG PET/CT scans and 0.8% of them received & GE; 100 mSv in a year more than one time, the CED of 27.8% (9087 patients) who underwent 2 to 23 F-18-FDG PET/CT scans over 11-years ranged from 38 to 575 mSv (median: 271 mSv). Conclusion This largest ever study covering analysis of 11-years' data of F-18-FDG PET/CT patients showed that a sizeable number of patients, largely with malignant disease (82%) undergo recurrent imaging during one year and non-ignorable fraction exceed 100 mSv in one year.