Orocaecal transit time was investigated using the hydrogen breath test in 39 insulin-requiring patients with long-standing Type I diabetes mellitus and 26 healthy control subjects. Thirty four patients complained of different gastrointestinal symptoms. The standard meal consisted of 10 g lactulose in 150 ml tap water. Mean transit time was significantly longer in the patient group (106.4+/-31.1 min) than in control subjects (84.2+/-27.1 min), and differences in OCTT between symptomatic subgroups were also significant. No correlation was found between orocaecal transit time and gastric emptying of a solid meal measured with scintigraphic method, HbA1c values, and other signs of automatic and peripheral neuropathy. The incidence of bacterial overgrowth among the diabetics was minimal. The percentage of H2 non-producers did not significantly differ between control and patient groups (23% and 26%, respectively). The absolute amount of breathed hydrogen was, however, significantly lower in diabetics at all time intervals. This indicates that specific changes in hydrogen production may be related to pathophysiological features as a consequence or as an associated symptom.