The relationship between gastric acid secretion and ulcer healing was analysed in 168 patients with endoscopically proven duodenal ulceration. One hundred and three patients received active treatment with various therapeutic agents and 65 received placebo. The mean maximal acid output in patients with unhealed ulcers was significantly higher than that in patients with endoscopically confirmed ulcer healing in both the actively treated and the placebo groups (P less than 0,001). In patients with normosecretory acid status the rate of ulcer healing on active treatment was significantly higher than that achieved with placebo (P less than 0,001). However, no significant difference in ulcer healing was evident following active treatment or placebo administration in patients with hypersecretion of acid. Acid secretory profiles warrant consideration when the results of clinical trials are interpreted.