AbstractA cancerostatic effect of vegetarian diets is proposed on the basis of a selective alteration of the metabolic pathways of fatty acids in neoplastic cells. Most vegetables lack the enzyme 6‐desaturase (6D), which converts linoleic to arachidonic acid. Human cells have 6D, and therefore humans do not need to eat the higher polyunsaturated fatty acids found in animal tissues. Many neoplastic cells have lost the activity of 6D. A vegetarian diet would deprive neoplastic cells of higher‐chain fatty acids and inhibit the activity of 6D. Without higher‐carbon fatty acids, neoplastic cell membranes would have altered fluidity and thus altered transport properties, receptor activity, sensitivity to external molecules, ability to reproduce, resistance to external agents (drugs, radiation, immune defenses, temperature), and overall survival. These alterations would make the cells easier prey for the self‐defense of the body or for attack with therapeutic agents. Thus, a vegetarian diet would alter the tumor cell lipid membranes and decrease neoplastic cell survival.