Up until about 15 years ago lectins were thought of as laboratory tools, useful for histochemistry and blood transfusion work. The fact that many common foods are rich sources of lectin was not considered by most biomedical scientists. In the last decade, however, there has been a flowering of knowledge about the interactions of lectins with body organs and tissues, and it has become clear that many lectins are resistant to cooking and digestion and are distributed to distant parts of the body after ingestion. There is now abundant evidence that dietary lectins can cause disease in Man and animals. This review will adduce evidence that such hitherto mysterious diseases as inflammatory bowel disease, diabetes mellitus, rheumatoid arthritis, glomerulonephri-tis, psoriasis, multiple sclerosis, retinitis and cataracts in the eye, are all explicable in terms of a lectin aetiology, as are congenital malformations, infertility, IgE-mediated allergies and autoimmune states. Complete proof is still lacking in most cases, but the new hypothesis, if true, offers scope for rational therapy in these hitherto intractable diseases