Sixteen patients with pentagastrin-fast achlorhydria and 12 patients who had undergone Billroth II gastrectomy (at least 3 years previously) were compared with 10 acid-secreting volunteers and 13 patients with endoscopically proven peptic disease. The concentration and type of gastric bacteria were analysed in achlorhydrics, Billroth II patients, and patients with peptic disease. A 6-h hydrogen (H2) breath test after a standardized meal was performed in all subjects. The mean concentration of gastric bacteria was significantly higher in achlorhydrics and Billroth II patients than in patients with peptic disease. End-expiratory H2 excretion was elevated in achlorhydrics and Billroth II patients to levels significantly exceeding those of acid-secreting volunteers and patients with peptic disease. In achlorhydrics, total bacterial concentration in gastric juice was correlated to H2 excretion between 60 and 180 min after the meal. Treatment of achlorhydric and postgastrectomy patients with trimethoprim/sulphamethoxazole lowered H2 breath concentrations in both groups and reduced symptoms in achlorhydrics. Elevated end-expiratory H2 levels after a test meal indicate upper gastrointestinal bacterial overgrowth in achlorhydrics and in postgastrectomy patients.