The mechanisms of chronic diarrhoea, a frequent symptom in diabetes mellitus, are multifactorial and complex, although small intestinal bacterial overgrowth and autonomic neuropathy seem to play a major role. This study evaluated the prevalence of small intestinal bacterial overgrowth and the effects of antibiotic treatment in a population of diabetic patients with chronic diarrhoea (defined as > 3 stools/24 h, weight > 200 g/24 h, duration > 3 weeks). Small intestinal bacterial overgrowth syndrome was diagnosed by glucose-hydrogen breath testing (sensitivity: 78%, specificity: 89%). The characteristics of diarrhoea (duration, number of stools per day, and gastrointestinal symptoms) were noted. Autonomic neuropathy was assessed by cardiac parasympathetic tests. A total of 35 patients were included, 15 with small intestinal bacterial overgrowth syndrome (43%, group 1) and 20 with no bacterial overgrowth (group 2). Age (52.9 +/- 13.5 vs. 53.9 +/- 11.8 years, NS), duration of diabetes (13.8 +/- 9.1 vs. 10.6 +/- 7.8 years, NS), and HbA1c level (10 +/- 2.9 vs. 10.9 +/- 2.4%, NS) were not different between the two groups. In group 1, duration of diarrhoea was longer (18.1 +/- 18.5 vs. 7.75 +/- 4.02 months, P = 0.05), the number of stools higher (7.1 +/- 5.7 vs. 4.6 +/- 2.6/24 h, P < 0.05), and gastrointestinal symptoms more frequent (13 vs. 10, P < 0.05). The prevalence of small intestinal bacterial overgrowth syndrome and gastrointestinal symptoms was not different in patients with and without autonomic neuropathy (9 vs. 8 and 12 vs. 11 respectively, NS). Eight patients with bacterial overgrowth received antibiotics (amoxicillin-clavulanic acid, 1.5 g/24 h for 10 days). Dramatic clinical improvement was observed in 6 out of 8 of these patients. It is concluded that small intestinal bacterial overgrowth should be considered in case of chronic diabetic diarrhoea because of its frequency (43%), facility of diagnosis, and often successful treatment with antibiotics.