The biological basis for the epidemiological association between high dietary fibre intakes and reduced disease incidence in Man is not fully understood. It is proposed that molecular hydrogen, produced in quite large amounts as a by-product of colonic fermentation of dietary fibre and unabsorbed carbohydrate may play an important role. Hydrogen in the tissues may act as a powerful reducing agent (antioxidant) of potentially damaging free radical species (e.g. singlet oxygen) and in so doing, along with other antioxidants would help in the control of these potentially damaging species and their known involvement in disease causation.