To determine the relationship between gastric secretion and gastric carcinoma, we investigated gastric acid secretion and the fasting serum levels of pepsinogen I and gastrin in 50 Japanese patients with early gastric carcinoma. After the histological and macroscopic type of carcinoma had been determined, results were compared with findings in 50 Japanese control subjects whose gastric mucosa was endoscopically normal. The maximum gastric acid secretion and fasting levels of serum pepsinogen I were significantly lower in intestinal type gastric carcinoma than in diffuse type carcinoma and in the controls. They were also significantly lower in the non-ulcerative (elevated or flat) type than in the ulcerative (depressed) type of carcinoma. The serum gastrin levels in patients with early gastric carcinoma of either the intestinal or diffuse type were higher than those in the control subjects, though the difference was not significant. Gastric acid secretion and serum pepsinogen I levels were related with both the histological and macroscopic types of gastric carcinoma. These findings suggest that the serum pepsinogen I level might be useful as a maker for early gastric carcinoma of the intestinal type.