The aetiology of colorectal cancer is multifactorial, with lifestyle having a major role; smoking (colon cancer only) (Secretan et al., 2009), alcohol (Secretan et al., 2009), obesity (International Agency for Research on Cancer, 2002), some aspects of diet and physical inactivity (World Cancer Research Fund/American Institute for Cancer Research, 2007) have been shown to be risk factors, while use of hormone replacement therapy and oral contraceptives (International Agency for Research on Cancer, 2011a) and non-steroidal anti-inflammatory drugs (International Agency for Research on Cancer, 1997) have been shown to be protective. RoI had a slightly higher risk for men than NI, when adjusted for factors such as population density and unemployment. The risk was correlated with population density and unemployment for men, but there was only a modest variation in risk across the island. The area of higher risk in Cork and along the south coast was common to men and women. An area of higher risk across most of NI (apart from the area around Belfast) and in Donegal, and of low risk in Connacht, was seen for women only while the areas of higher risk in Dublin, Meath and Louth were seen for men only . As the incidence of colorectal cancer is considerably higher in men than in women, the prevalence of important aetiological factors seems to be different for the two sexes, or at least to affect risk in different ways. One difference suggested has been the protective effect of hormone replacement therapy and oral contraceptives (dos Santos Silva and Swerdlow, 1996). We do not have enough information on the geographical variation of these factors between the sexes to be able to interpret the geographical patterns shown; however the higher overall female risk in rural NI and around Cork is worthy of further investigation.
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