Lewis system carbohydrate antigens have been shown to be expressed at high levels in many cancers of epithelial cell origin, including those of colon, breast, lung, prostate and ovary. The type 1 (Le(a) and Le(b)) antigens are important histo-blood groups, while type 2 (Le(x) and Le(y)) antigens in healthy individuals are only expressed, at relatively low levels, by a few tissues, including some epithelial cells. Thus, the type 2 antigens are considered to be tumour-associated antigens and are promising targets for cancer treatment, including antibody-based immunotherapy. In this review, we discuss the conformational characteristics of the free and bound forms of Lewis oligosaccharides and the 3D structures of antibodies in complex with Le(y) and Le(x) antigens. Collectively, the structural studies have demonstrated that the Lewis determinants are rigid structures, which generally maintain the same conformation in the free and bound states. The rigid nature and similarities in shape of type 1 and 2 Lewis oligosaccharides appear to make them perfectly suited to driving a structurally convergent immune response (at least in the case of Le(y) specific antibodies) toward a highly specific recognition of individual carbohydrate determinants, which is a goal in the development of effective antibody-based cancer treatments.