Patients with Candida colonization and invasion were studied in 20 duodenal ulcer (DU) patients, 10 non-ulcer dyspepsia (NUD) cases and 10 controls with non-gastroenterological disorders. Of 20 DU patients, Candida albicans was cultured from duodenal aspirates of 6 (30%) DU patients and pseudohyphae, indicating invasive form, were present in two (10%) prior to treatment. NUD patients and controls had positive Candida culture in 3 (30%) and 1 (10%) cases respectively; pseudohyphae were found in none. Six DU patients (30%) failed to heal with 4 weeks of cimetidine therapy; all six showed invasion of duodenal ulcer mucosa with Candida on histology. The pseudohyphae form of Candida was always found to be associated with non-healing of DU. Candidal invasion is associated with failure to respond to, H2-receptor antagonist therapy.