The sensitivity and specificity of the preformed urease test and of carbolfuchsin-stained smears for the diagnosis of the presence of Helicobacter pylori in gastric mucosa were evaluated before and after antimicrobial treatment. The results obtained by culture were used as the reference point. We studied 41 patients with endoscopically diagnosed duodenal ulcer. Twenty-five of these were treated with furazolidone (100 mg t.i.d.), amoxicillin (500 mg t.i.d.) and metronidazole (250 mg t.i.d.) for 5 days and then with only furazolidone (100 mg t.i.d.) for an additional 25 days. The 16 control patients were treated with cimetidine (800 mg, 4 times a day). The sensitivity of the urease test and of direct smear examination was 100% before treatment and 84.6% and 92.3%, respectively, after treatment. We conclude that the urease test and carbolfuchsin-stained smears, which are highly sensitive for H. pylori diagnosis, present reduced sensitivity when they are employed for the follow-up of patients treated with antimicrobials.