Background: Colorectal cancer is generally diagnosed following a symptomatic presentation to primary care. Although the presenting features of the cancer are well described, the risks they convey are less well known. This study aimed to quantify the risk of cancer for different symptoms, across age groups and in both sexes. Methods: This was a case-control study using pre-existing records in a large electronic primary care database. Cases were patients aged 30 years or older with a diagnosis of colorectal cancer between January 2001 and July 2006, matched to seven controls by age, sex and practice. All features of colorectal cancer recorded in the 2 years before diagnosis were identified. Features independently associated with cancer were identified using multivariable conditional logistic regression, and their risk of cancer quantified. Results: We identified 5477 cases, with 38,314 age, sex and practice-matched controls. Six symptoms and two abnormal investigations (anaemia and microcytosis) were independently associated with colorectal cancer. The positive predictive values of symptoms were: rectal bleeding, positive predictive value for a male aged >= 80 years 4.5% (95% confidence interval 3.5, 5.9); change in bowel habit 3.9% (2.8, 5.5); weight loss 0.8% (0.5, 1.3); abdominal pain 1.2% (1.0, 1.4); diarrhoea 1.2% (1.0, 1.5) and constipation 0.7% (0.6, 0.8). Positive predictive values were lower in females and younger patients. Only 27% of patients had reported either of the two higher risk symptoms. Conclusion: Most symptomatic colorectal cancers present with only a low-risk symptom. There is a need to find a method of identifying those at highest risk of cancer from the large number presenting with such symptoms.