Background Modifiable behavioural risk factors—including exercise, obesity and smoking—have been causally associated with colorectal cancer mortality. However, results have been inconsistent and undiagnosed cancers may affect baseline risk factors, distorting the temporal relationship that is observed between them.

Objective To determine whether risk factors for colorectal cancers available in the Whitehall I study were predictive of colonic or rectal cancer mortality.

Methods Prospective cohort study over 40 years on Whitehall I men aged 40–69 on entry between 1967 and 1970. Associations between baseline risk factors and cause-specific mortality were tested with Cox proportional hazards models. Events within the first 10 years of follow-up were excluded to minimise ‘reverse causality.’

Results 329 colon and 121 rectal cancer deaths occurred among 17 949 men followed up for a total of 472 523 person-years. Age and smoking were associated with increased mortality from colorectal cancers. Compared with never-smokers, current smoking was associated with age-adjusted HRs for colon and rectal cancers of 1.45 (95% CI 1.03 to 2.03) and 1.97 (95% CI 1.02 to 3.80), respectively. A significant effect of current smoking on rectal cancer mortality was only apparent after events in the first 10 years of follow-up were excluded. No convincing evidence was found that body mass index, diabetes mellitus, blood pressure or physical activity were associated with colorectal cancer mortality.

Conclusion Smoking significantly increases mortality from colorectal cancer and its decreasing prevalence in the UK may partly explain falling mortality from the disease. Changes in health behaviours in response to early cancer symptoms may result in differential misclassification or ‘reverse causality’ unless early events are excluded. Although many individual cohort studies have not shown significant relationships between behavioural risk factors and colorectal cancer mortality, their contribution to meta-analyses remains important.