The possibility of delaying gastric emptying and improving lactose digestion and tolerance by increasing milk viscosity was studied in 13 lactose maldigesters who ingested three test milks with different viscosities (range: 33-1892 mPa.s) in random order at intervals of 1 wk. Each test portion was 500 mL and provided approximately equal to 1900 kJ and 18 g lactose. The different viscosities were obtained by adding varying proportions of rice starch and maltodextrin to a basic milk formula. A combined [13C]glycine-hydrogen breath test was used to measure gastric emptying and lactose digestion simultaneously. Participants reported their gastrointestinal symptoms by using a four-grade scale. Mean (+/- SEM) gastric-emptying half times were 78 +/- 5.7 min for low-viscosity milk (30 mPa.s), 86 +/- 5.0 min for moderate-viscosity milk (80 mPa.s), and 78 +/- 4.5 min for high-viscosity milk (1.9.10(3) mPa.s). Mean orocecal transit times (180 +/- 24, 163 +/- 23, and 180 +/- 24 min, respectively) were not significantly different. There were no milk-dependent differences in breath-hydrogen excretion or in the severity of gastrointestinal symptoms. The milks were well tolerated; > 50% of the subjects reported nondisturbing symptoms or none. We conclude that gastric emptying, orocecal transit time, and lactose digestion and tolerance were not affected by altering milk viscosity. This may have been due to the high energy content of the test milks, which in itself led to slow gastric emptying.