Introduction: Colonoscopy and PET / CT are among the major diagnostic tests for colorectal cancer. The sensitivity, specificity, and accuracy of these studies are still being assessed differently. Objective: The aim of the study was to compare the sensitivity, specificity and accuracy of colonoscopy and PET / CT in the diagnosis of colorectal cancer. Material and methods: The medical records of 125 patients with colonoscopy and PET / CT in the years 2014-2015 were analyzed retrospectively. The research was done at the Professor Franciszek Łukaszczyk Oncology Center in Bydgoszcz. Based on the macroscopic description of colonoscopy, the results were divided into two groups: with and without probability of cancer. The average SUV value in PET / CT for colorectal cancer was calculated and without this diagnosis. The average value of SUV 14 and higher was considered probable, while 11 or less had no probability of cancer. Standardized mathematical formulas were used to evaluate the sensitivity, specificity and accuracy. Results: More than half of the patients - 78 (62.4%) were males. The majority of patients -42 (36.6%) were aged 65-74. The majority (106) (68.8%) were diagnosed as polyps and 24 (15.6%) as tumor-like lesions. Polyps were placed in the rectum -32 (30.2%), in the sigmoid colon - 26 (24.5%) and 15 (13.2%) in the ascending colon. Tumors were located in the rectum - 11 (45.8%) and 4 (16.7%) in the recto-sigmoid junction. 38 (24.6%) adenocarcinomas and 67 (43.5%) adenomas were diagnosed. The detection rate of RJG was 32% in colonoscopy and PET / CT. The sensitivity of the colonoscopy was 80%, the specificity - 68.4% and the accuracy - 71.4%. The sensitivity, specificity and accuracy of PET / CT were 65%, 75%, 4% and 72.7%, respectively. Conclusions: Colonoscopy has a higher sensitivity in colorectal cancer diagnosis, but specificity and accuracy are higher in PET / CT. Keywords: colorectal cancer, colonoscopy, PET / CT, sensitivity, specificity, accuracy