BACKGROUND: The objective of this study is to investigate symptoms, clinical factors and socio-demographic factors associated with colorectal cancer (CRC) diagnosis and time to diagnosis. METHODS: Prospective cohort study of participants referred for suspicion of CRC in two English regions. Data were collected using a patient questionnaire, primary care and hospital records. Descriptive and regression analyses examined associations between symptoms and patient factors with total diagnostic interval (TDI), patient interval (PI), health system interval (HSI) and stage. RESULTS: A total of 2677 (22%) participants responded; after exclusions, 2507 remained. Participants were diagnosed with CRC (6.1%, 56% late stage), other cancers (2.0%) or no cancer (91.9%). Half the cohort had a solitary first symptom (1332, 53.1%); multiple first symptoms were common. In this referred population, rectal bleeding was the only initial symptom more frequent among cancer than non-cancer cases (34.2% vs 23.9%, P=0.004). There was no evidence of differences in TDI, PI or HSI for those with cancer vs non-cancer diagnoses (median TDI CRC 124 vs non-cancer 138 days, P=0.142). First symptoms associated with shorter TDIs were rectal bleeding, change in bowel habit, 'feeling different' and fatigue/tiredness. Anxiety, depression and gastro-intestinal co-morbidities were associated with longer HSIs and TDIs. Symptom duration-dependent effects were found for rectal bleeding and change in bowel habit. CONCLUSIONS: Doctors and patients respond less promptly to some symptoms of CRC than others. Healthcare professionals should be vigilant to the possibility of CRC in patients with relevant symptoms and mental health or gastro-intestinal comorbidities.