BACKGROUND: A lactose-free diet is commonly prescribed to subjects with hypolactasia. We tested the effectiveness of a single ingestion of tilactase (a beta-D-galactosidase from Aspergillus oryzae) in adults with hypolactasia, previously assessed by lactose H(2)-breath test. MATERIALS AND METHODS: After measurement of orocecal transit time (OCTT, by lactulose H(2)-breath test) and lactose H(2)-breath testing plus placebo, a total of 134 subjects were positive to hypolactasia and underwent lactose H(2)-breath testing plus either low (6750 U) or standard (11,250 U) doses of tilactase. The appearance of gastrointestinal symptoms during the tests was monitored. RESULTS: OCTT was longer in malabsorbers (subjects without bloating, abdominal pain and/or diarrhoea, n = 25) than in intolerants (bloating, abdominal pain and/or diarrhoea, n = 109, P