Of 212 patients with duodenal ulcer treated with four weeks of one gram daily cimetidine, 25 had ulcers which underwent no reduction in size despite treatment. The effects of tripotassium dicitrato bismuthate (TDB) tablet four times a day or cimetidine 1.6 g daily on the healing of these cimetidine resistant ulcers were compared in a randomised crossover trial. Ten of 12 patients on tripotassium dicitrato bismuthate and five of 13 patients on high dose cimetidine had complete healing (p less than 0.02). On crossing over, seven of the eight ulcers not healed by high dose cimetidine completely healed with TDB in another four weeks, and one of the two ulcers not healed by TDB healed with high dose cimetidine. Overall, TDB healed 85% of cimetidine resistant ulcers, whereas high dose cimetidine healed 40% (p less than 0.006). Tripotassium dicitrato bismuthate is recommended for cimetidine resistant duodenal ulcers.