The present study was designed to investigate the prevalence of bacterial overgrowth in patients with exocrine pancreatic insufficiency by using the hydrogen breath test with glucose. Thus, in 30 patients with exocrine pancreatic insufficiency (in 15 due to chronic pancreatitis and in 15 associated to primary immunodeficiency), established by quantifying trypsin output before and after stimulation with cerulein using a duodenal perfusion technique, a glucose test was performed by administering 50 g of glucose and quantifying H2 in the breath by gas chromatography. The glucose test was positive in six of 15 patients with chronic pancreatitis but in only one of 15 immunodeficient patients (p < 0.05). Age, sex, etiology, time of evolution, associated diabetes, pancreatic calcifications, duodenal pH, or duodenal trypsin output did not differ between patients with and those without bacterial overgrowth. Previous gastroduodenal surgery was more common in chronic pancreatitis patients with overgrowth (six of six vs. four of nine; p < 0.05). Five patients with a positive glucose test were treated with antibiotics for 2 weeks and became negative in two of them. These results suggest that a positive glucose test indicating overgrowth is relatively common in exocrine pancreatic insufficiency due to chronic pancreatic, especially in patients with previous gastroduodenal surgery.