Background: The prognosis of colorectal cancer (CRC) patients is determined to a decisive extent by comorbidities. On the other hand, anti-cancer treatments for CRC are associated with relevant toxicities and may therefore cause additional comorbidities. Methods: This retrospective cohort study assessed the prevalence of various diseases in patients 12 months before and 12 months after an initial diagnosis of colorectal cancer (ICD-10: C18, C20) in 1274 general practices in Germany between January 2000 and December 2018. The study is based on the Disease Analyzer database (IQVIA), which contains drug prescriptions, diagnoses, and basic medical and demographic data. Patients with and without CRC were matched by sex, age, and index year. Results: We identified several diagnoses with a significantly higher prevalence among CRC patients 12 months prior to the index date compared to controls. These diagnoses included gastrointestinal hemorrhage, hemorrhoids, perianal venous thrombosis, and abdominal and pelvic pain, as well as functional intestinal disorders. In contrast, the prevalence of lipid metabolism disorder, depression, hypertension, coronary heart disease, or acute bronchitis was significantly lower in CRC cases. After diagnosis of CRC, we found a significantly higher prevalence of anemia, polyneuropathies, functional intestinal disorders, and chronic kidney disease among CRC patients compared to the control group, while the prevalence of acute upper respiratory infections of multiple and unspecified sites and acute bronchitis was significantly lower in CRC patients compared to non-CRC patients. Conclusions: In the present study, we identified a variety of diseases occurring at higher or lower frequencies in CRC patients compared to matched controls without CRC. This might help to select patients for early CRC screening and improve the clinical management of CRC patients.