Purpose: Accurate staging of colonic cancer is important for patient stratification. We aimed to correlate the diagnostic accuracy of preoperative computed tomography (CT) with final histopathology as reference standard. Methods: Data was collected retrospectively on 615 consecutive patients operated for colonic cancer. Evaluation was based upon T-stage. Patients were stratified into high-risk and low-risk groups, based on the extent of tumor invasion beyond the proper muscle layer of more or less than 5 mm. The Kendall tau correlation coefficient was used to calculate concordance between radiological (r)T-stage obtained at CT imaging and pathological (p)T-stage from the final pathology. Results: In total, 501 patients were included. We found no significant differences in the Kendall tau values for diagnostic measures between the groups at the 95% confidence interval (CI) level: 49% (95% CI, 43–55) for all individuals, 48% (95% CI, 40–56) for screened individuals, and 47% (95% CI, 37-56) for non-screened individuals. The overall sensitivity and specificity for all individuals in identifying high-risk tumors on CT was 65% (95% CI, 56–73) and 89% (95% CI, 85–92). The risk of ending up in the high-risk group due to overstaging among all individuals was calculated as the number needed to harm 11.7 (95% CI, 9–16). Conclusions: There is basis for improvement of CT-based preoperative staging of patients with colorectal cancer. Supplementary modalities may be needed for correct staging of patients preoperatively, especially in relation to stratification of patients into neoadjuvant treatments or tailored therapy in patients with early cancers.