A European multicentric intervention study, led by the colon group of the European Cancer Prevention Organization, is under way. The main aim of the study is to test the efficacy of oral calcium supplementation with 2 g calcium per day and oral dietary supplementation with mucilaginous substances (as 3.8 g of ispaghula husk) on adenoma recurrence. Secondary aims are the study of treatment efficacy on colonic cell proliferation and on stool bile acid and sterol concentration. Serum and plasma samples are also collected. To better interpret the effect of the intervention, a diet history questionnaire and an aspirin and anti-inflammatory drug questionnaire are administered. The aim will be achieved through a randomized placebo-controlled clinical trial using a parallel design in patients aged 35 to 75 at entry with a complete colonoscopy and a clean colon. Overall, 655 subjects have been included. All randomized patients are followed up every six months for 3 years. If one of the evaluated interventions proves efficient, the benefits of a simple, safe and inexpensive prophylaxy for a very common cancer will be clear.