Following a review of the literature, the relationship between diet and the onset of colorectal cancer is analysed starting from the consideration that in Italy about 20,000 people every year from carcinoma of the colon and 50% of these do not survive. The authors proceed to analyse the epidemiological data which point to diet as an aetiological factor in the cancerogenesis of a variety of tumours in spite of the fact that none of the individual nutritional components has been specifically identified as a triggering and/or protective agent, with the exception perhaps of alcohol in the cancer-cirrhosis sequence. They conclude by stating that, while continuing to give the correct importance to integrated surgical, chemo and radio treatment, to prevent the onset of tumours of the large intestine it is useful to associate the support of a complete nutritional education, which should be begun as soon as possible, with the canonical screening techniques. This educational programme should stress the importance of diet as a contributor of protective principles such as fruit, vegetables, vitamins and non-absorbable fibres which reduce contact time between the carcinogenic substances derived from a prevalently meat diet (cholesterol stimulates the production of biliary salts by increasing the quota of taurodesoxycolic and lithocolic acid and other carcinogenic factors represented by conservants and chemical additives such as nitrates and nitrites which can have a carcinogenetic activity) and the intestinal mucosa.