The outcome of every cancer illness is determined to a significant extent by the inherent resistance of the individual patient to his or her disease. There is increasing recognition that resistance to cancer depends, to a certain extent, upon the availability of certain nutritional factors, of which ascorbic acid appears to be the most important. Ascorbic acid protects against the destructive effects of malignant invasiveness by stabilizing the ground substance and enhancing collagen encapsulation. It is also involved in the mechanisms of both cell-mediated and humoral immunity including the production of interferon, and other factors either known or thought to be involved collectively in host resistance to neoplasia. There is also some recent evidence suggesting that ascorbate exerts a selective cytotoxic effect against malignant cells. All these observations indicate that supplemental ascorbate should be of some therapeutic value in the treatment of cancer. Clinical trials conducted over the last decade are briefly reviewed, and the balance of evidence supports this contention. It is predicted that in the not-to-distant future, supplemental ascorbate will attain an established place in all comprehensive cancer treatment regimes.