A pathogenetic mechanism to explain the increased incidence of gastric cancer in patients with pernicious anaemia (P.A.) is proposed. Mean nitrite concentration in gastric juice from thirteen fasting patients with P.A. was nearly fiftyfold greater than that of age-matched controls. The number of bacteria in the gastric juice of P.A. patients was also greatly increased. Small amounts of volatile nitrosamines were detected in simulated P.A. gastric juice in vitro, after addition of nitrite to achieve a concentration approximating to that found in vivo. If similar nitrosation occurs in vivo, the intragastric production of carcinogenic N-nitroso compounds could explain the high incidence of gastric cancer in patients with P.A.